Bhadra IAS academyNotes MODULE - 3 Moving Things 191 Motion and its Description SCIENCE AND...

Notes

MODULE - 3Moving Things

191

Motion and its Description

SCIENCE AND TECHNOLOGY

9

MOTION AND ITS DESCRIPTION

You must have seen number of things in motion. For example car, bicycle, bus movingon a road, train moving on rails, aeroplane flying in the sky, blades of an electricfan and a child on a swing. What makes things move? Are all the motions similar?

You might have seen that some move along straight line, some along curved pathand some to and fro from a fixed position. How and why these motions are different?You will find answers to all such questions in this lesson. Besides studying aboutvarious types of motions, you will learn how to describe a motion. For this we willtry to understand the concepts of distance, displacement, velocity and acceleration.We will also learn how these concepts are related with each other as well as withtime. How a body moving with constant speed can acquire acceleration will also bediscussed in this lesson.

OBJECTIVES

After completing this lesson you will be able to:

� explain the concept of motion and distinguish between rest and motion;

� describe various types of motion – rectilinear, circular, rotational andoscillatory;

� define distance, displacement, speed, average speed, velocity andacceleration;

� describe uniform and uniformly accelerated motion in one dimension;

� draw and interpret the distance time graphs and velocity time graphs;

� establish relationship among displacement, speed, average speed, velocityand acceleration;

� apply these equations to make daily life situation convenient and

� explain the circular motion.

Notes


MODULE - 3 Motion and its Description

Moving Things

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9.1 MOTION AND REST

If you observe a moving bus you will notice that the position of bus is changing withtime. What does this mean? This means that the bus is in motion. Now suppose youare sitting in a bus moving parallel to another bus moving in the same direction withsame speed. You will observe that the position of the other bus with respect to yourbus is not changing with time. In this case the other bus seems to be at rest withrespect to your bus. However, both the buses are moving with respect tosurroundings. Thus, an object in motion can be at rest with respect to one observerwhereas for another observer, the same object may be in motion. Thus we can saythat the motion is relative.

Let us understand the concept of relative motion. Suppose you are sitting in a vehiclewaiting for traffic signal and the vehicle beside you just starts moving, you will feelthat your vehicle is moving backward.

Suppose Chintu and Golu are going to the market. Golu is running and Chintu iswalking behind him. The distance between the two will go on increasing, though bothare moving in the same direction. To Golu it will appear that Chintu is moving awayfrom him. To Chintu also, it will appear that Golu is moving ahead and away fromhim. This is also an example of relative motion. See Fig. 9.1.

Fig. 9.1 An example of relative motion

Think and Do

One day, Nimish while standing on the bank of a river in the evening observedboats were approaching the bank, vehicles passing on the bridge, cattle goingaway from the bank of the river towards the village, moon rising in the sky, birdsflying and going back to their nests, etc. Can you list some thoughts that couldbe emerging in the mind of the Nimish. What type of world Nimish has aroundhim?

Chintu Golu

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A

C

B

DE

We can conclude that motion is a continuous change in the position of the objectwith respect to the observer. Suppose you are moving towards your friend standingin a field. In what way are you in motion? Are you in motion if you are observingyourself? Is your friend in motion with respect to you? Are you in motion with respectto your friend? Now you may have understood that observer with respect to itselfcan not be in motion. Thus, you are moving towards the object with respect to yourfriend and your friend is moving towards you with respect to you in opposite direction.In other words the change in position of the object with respect to observer decideswhether object is in motion. This change should also be continuous. Let us take aninteresting example to understand the concept of motion. There are five playersparticipating in 200 metre race event. They are running in their lanes as shown inthe Fig. 9.2. The players A, B, C, D and E runs 2, 3, 4, 3, 2 metre respectivelyin one second. Can you help the player to understand that which player is in motionwith respect to which player and which player is at rest with respect to which player?Fill your responses in the table given below.

Fig. 9.2

Table 9.1

Observer player Player in motion Player at rest Remark

A B, C, D E E is in rest with respect toA because change inposition of A and E in1second is zero while inother cases is not.

B

C

D

Now you will be able to help Nimish to answer some of his questions.

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9.1.1 Types of Motion

In our daily life we see many objects moving. Some objects moving in straight lineand some are not. For example, a ball rolls on a horizontal surface, a stone fallingfrom a building, and a runner on 100 m race track. In all these examples, you maynotice that the position of moving objects is changing with respect to time along astraight line. This type of motion is called motion in a straight line or rectilinearmotion.

Fig. 9.3 Example of rectilinear motion

Can you think at least two more other example of such motions. You might haveobserved the motion of time hands of a clock, motion of child sitting on a merry-go-round, motion of the blades of an electric fan. In such a motion, an object followsa circular path during motion. This type of motion is called circular motion.

ACTIVITY 9.1

(A) Suspend a small stone with a string (of length less than your height) with thehelp of your hand. Displace the stone aside from the position of rest and release.

(B) Let the stone comes to rest and bring it to the point of suspension with the helpof your hand and release it.

(a) Ball rolling on horizontal surface

(b) Stone falling by hand

(c) A runner on a 100 m race track

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(A)

(B)

(C)

(C) Now hold the stone firmly in your hand and whirl it over your head.

Write in table given below, what type of motion of stone you have observed in allthe above three cases with justification.

Table 9.2

Case Type of motion Justification

A

B

C

(A) A person suspend the stone attached to a string, (B) A person oscillate the stoneattached to a string, (C) A person whirling the stone attached to a string

Fig. 9.4 (A), (B) (C)

Have you ever noticed that the motion of the branches of a tree? They move to andfro from their central positions (position of rest). Such type of motion is calledoscillatory motion. In such a motion, an object oscillates about a point often calledposition of rest or equilibrium position. The motion of swing and pendulum of wallclock are also oscillating motions. Can you think about the motion of the needle ofa sewing machine? What type of motion is it? Now you can distinguish some of themotions viewed by Nimish.

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9.2 DISTANCE AND DISPLACEMENT

For a moving object two points are significant. One is the point of start or originwhere from the object starts its motion and the other is the point where it reachesafter certain interval of time. Points of start and destination are connected by a pathtaken by the object during its motion. The length of the path followed by object iscalled distance. There may be a number of paths between the point of start and thepoint of destination. Hence the object may cover different distances between samepoint of start and destination. The unit of distance is metre (m) or kilometre (km).

ACTIVITY 9.2

An object moves from point A to B along three different paths. Measure the distancetravelled by object along these three paths.

Fig. 9.5

In any motion, you will notice that object gets displaced while it changes its positioncontinuously. The change in position of the object is called displacement.Basically, it is the shortest distance between initial and final position of the object.The path followed by the object between initial and final positions may or may notbe straight line. Hence, the length of the path does not always represent thedisplacement.

ACTIVITY 9.3

In the following cases measure the distance and displacement and write their valuesin the table given below:

(a) A body moves from A to B

(b) A body moves from A to B then comes to C

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(c) A body goes from position A to B and comes back to position A

(d) A body goes from posion A to B and then C

(e) A body moves from position A to B along a circular arc

Fig. 9.6

Table 9.3

Case Distance Displacement

(i)

(ii)

(iii)

(iv)

(v)

Now you can conclude that:

(a) displacement is smaller or equal to the distance.

(b) displacement is equal to distance, if body moves along a straight line path anddoes not change its direction.

(c) if a body does not move along a straight line path its displacement is less thanthe distance.

(d) displacement can be zero but distance can not be zero.

(e) magnitude of displacement is the minimum distance between final position andinitial position.

(f) distance is the length of the path followed by the body.

(g) distance is path dependent while displacement is position dependent.

Can you now, suggest a situation in which the distance is twice the displacement?

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9.2.1 Graphical Representation of Distance and Displacement

Distance and displacement can also be shown by graphical representation. To drawa graph, follow the following steps:

(i) Analyse the range of variables (maximum and minimum values).

(ii) Select the suitable scale to represent the data on the graph line adequately.

(iii) Take independent quantity on x-axis and dependent quantity on y-axis.

Take distance on x-axis and displacement on y-axis. You know that for a motionalong a straight line without changing its direction the distance is always equal to thedisplacement. If you draw the graph, you will find that the graph line is a straightline passing through origin making an angle of 45° with distance axis as shown inFig 9.7.

Fig. 9.7

Let us take another situation where an object moving from one position to anotherand coming back to the same position. In this case the graph line will be a straightline making an angle of 45° with distance axis upto its maximum value and then comesto zero as shown in Fig. 9.8.

Fig. 9.8

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Now you can infer that:

� If graph line is a straight line making an angle of 45° with x-axis or y-axis, themotion is straight line motion and distance is equal to the displacement.

� For same value of displacement, the distance travelled can be different.

� If graph line does not make an angle of 45° with x-axis or y-axis, the motionwill not be straight line motion.

When an object moves along a circular path, the maximum displacement is equalto the diameter of the circular path and the distance travelled by object keeps onincreasing with time as shown in Fig. 9.9.

Fig. 9.9

INTEXT QUESTIONS 9.1

Choose the correct answer in the followings:

1. For an object moving along a straight line without changing its direction the

(a) distance travelled > displacement

(b) distance travelled < displacement

(c) distance travelled = displacement

(d) distance is not zero but displacement is zero

2. In a circular motion the distance travelled is

(a) always > displacement

(b) always < displacements

(c) always = displacement

(d) zero when displacement is zero

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3. Two persons start from position A and reach toposition B by two different paths ACB and ABrespectively as shown in Fig. 9.10.

(a) Their distances travelled are same

(b) Their displacement are same

(c) The displacement of I > the displacement of II

(d) The distance travelled by I < distance travelled by II

4. In respect of the top point of the bicycle wheel of radius R moving along astraight road, which of the following holds good during half of the wheel rotation.

(a) distance = displacement

(b) distance < displacement

(c) displacement = 2R

(d) displacement = πR

5. An object thrown vertically upward to the height of 20 m comes to the handsof the thrower in 10 second. The displacement of the object is

(a) 20 m (b) 40 m (c) Zero (d) 60 m

6. Draw a distance-displacement graph for an object in uniform circular motion ona track of radius 14 m.

9.3 UNIFORM AND NON-UNIFORM MOTION

Let us analyze the data of the motion of two objects A and B given in the table 9.4.

Table 9.4

Time in seconds (t) 0 10 20 30 40 50

Position of A (x1 in metre) 0 4 8 12 16 20

Position of object B (x2 in metre) 0 4 12 12 12 20

Do you find any difference between the motion of object A and B? Obviously objectsA and B start moving at the same time from rest and both objects travel equal distancein equal time. However, the object A has same rate of change in its position andobject B has different rate of change in position. The motion in which an object coversequal distance in equal interval of time is called uniform motion whereas the motionin which distance covered by object is not equal in equal interval of time is callednon-uniform motion. Thus, the motion of object A is uniform and of object B is non-uniform. You can draw the position-time graph for the motion of object A and B andobserve the nature of the graph for both types of motion.

For the uniform motion of object A the graph is a straight line graph and for non-uniform motion of object B the graph is not a straight line as shown in the Fig. 9.11.

Fig. 9.10

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Fig. 9.11 Graph representing uniform and non-uniform motion

9.3.1 Speed

While you plan your journey to visit a place of your interest you intend to think abouttime of journey so that you can arrange needful things like eatables etc. for that periodof time. How will you do it? For this you would like to know how far you haveto reach and how fast you can cover the destination. The measure of how fast motioncan take place is the speed. Speed can be defined as the distance travelled bya body in unit time.

Thus speed =Distance travelled

time takenIts SI unit is metre per second which is written as ms–1. The other commonly usedunit is km h–1.

i.e., 1 kmh–1 = –11000 m 5ms

60 60 s 18=

×

ACTIVITY 9.4

Here position of four bodies A, B, C and D are given after equal interval of timei.e. 2 s. Identify the nature of the motion of the bodies as uniform and non-uniformmotion.

Table 9.5

Time (s) → Bodies ↓ 0 2 4 6 8

positions (m) → A 0 4 8 12 16

B 0 8 8 10 12

C 4 8 12 16 20

D 0 6 12 16 20

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To identify the nature of the motion you can make a table as given below

Table 9.6

Time taken by body (s) → 2 – 0 = 2 4 – 2 = 2 6 – 4 = 2 8 – 6 = 2Distance covered by

body (m) ↓

A 4 – 0 = 4 8 – 4 = 4 12 – 8 = 4 16 – 12 = 4

B 8 – 0 = 8 8 – 8 = 0 10 – 8 = 2 12 – 10 = 2

C 8 – 4 = 4 12 – 8 = 4 16 – 12 = 4 20 – 16 = 4

D 8 – 4 = 4 12 – 6 = 6 16 – 12 = 4 20 – 16 = 4

From the above table you can conclude that body A and C travel equal distancesin equal interval of time so their motion is uniform. But the distances travelled bybody B and D for equal intervals of time are not equal, hence their motion is non-uniform motion.

To analyze the motion as uniform motion or non-uniform motion, the motion can berepresented by graph. The position-time graph of all the four bodies A, B, C andD is shown in Fig. 9.12.

Fig. 9.12

Now you can see that the bodies having uniform motion e.g. A and C have theirgraph line straight and the bodies having non-uniform motion do not have their positiontime graph line straight. In this graphical representation on axis 1 div = 1s and ony-axis 1 div = 2m.

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A graph drawn for different distances travelled by object with respect to time is calleddistance-time graph as shown in Fig. 9.13.

Fig. 9.13

In Fig. 9.13 distance travelled in 10 s is 22 m. Therefore, the speed of the object

=( )( )

22 m

10 s = 2.2 ms–1

This motion can be represented by another way i.e., speed = AB

OB. This ratio is also

known as slope of the graph line. Thus the speed is the slope of position-time graph.

Example 9.1 An object moves along a rectangular path of sides 20 m and 40 mrespectively. It takes 30 minutes to complete two rounds. What is the speed of theobject?

Solution:( )2 2 20 40 mDistance travelled

time taken 30 60 s

× +=

×

–14ms

30=

9.3.2 Velocity

If you are asked to reach a destination and you are provided three, four paths ofdifferent lengths, which of the path would you prefer? Obviously, the path of shortestlength but not always. This is also called displacement. In the previous section youhave learnt about distance. When motion is along the shortest path, it is directed fromthe point of start to the point of finish. How fast this motion is determines the velocity.The velocity is the ratio of length of the shortest path i.e. displacement to the timetaken

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velocity =Displacement

Time taken

Velocity has same unit as the unit of speed i.e., ms–1 (S.I. unit) or kmh–1.

The shortest path or the displacement is directed from initial position of the objectto the final position of the object. Hence, the velocity is also directed from initialposition of the object to the final position of the object. Thus we can say that thevelocity has direction. Speed does not have direction because it depends upon thetotal distance travelled by the object irrespective of the direction. The quantities whichhave direction are called vector and which do not have direction are called scalarquantity. Thus, velocity can also be expressed as

velocity =Change in position

Time taken

ACTIVITY 9.5

Observe the motion of an object in the following situations. Find speed and velocityin each situation and comment over the situation which you find different from other.

Ohject moves from A to B in time 10 s on the scale 1 cm = 10 m

Object moves from A to B than to C in 10 s on the scale 1 cm = 10 m

Object moves from A to B than to C in 20 s on the scale 1 cm = 10 m

Object completes a round of radius 7 m in 10 s

Fig. 9.14

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Now you will be able to distinguish the speed and velocity. Magnitude ofinstantaneous velocity is the speed. Now you can understand the importance ofpreplanning your journey to save time, effort and fuel etc.

Example 9.2 In a rectangular field of sides 60 m and 80 m respectively two formersstart moving from the same point and takes same time i.e. 30 minutes to reachdiagonally opposite point along two different paths as shown in Fig. 9.15. Find thevelocity and speed of both the formers.

Fig. 9.15

Solution: The displacement of both the former in same i.e.,

2 260 80+ = 3600 6400 10000+ = = 100 m

∴ Velocity A and B, v =displacement

time taken =

100 m 1

30 60s 18=

× ms–1

speed of A =( )80+60 mDistance travelled

time taken 30×60s=

=140

3800 ms–1 =14

18 ms–1

and speed of B =Distance travelled 100s

time taken 30×60s= =

1

18 ms–1

Note: In this example you can appreciate that the velocity of both the formers issame but not the speed.

9.3.3 Average speed and average velocity

Speed during a certain interval of time can not be used to determine total distancecovered in given time of the journey and also the time taken to cover the total distance

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of journey. It is because a body does not always travel equal distance in equal intervalof time. In most of the cases the body travels non-uniformly. Thus, in case of non-uniform motion to determine average speed is quite useful. The average speed canbe determined by the ratio of total distance covered to the total time taken.

Average speed =total distance covered

total time taken

Similarly in case of average velocity in place of total distance covered you can taketotal displacement.

∴ Average speed =total displacement

total time taken

Let us take few examples to understand the average speed and average velocity.

Example 9.3 If a body covers 50 m distance in 30 s and next 100 m in 45 s thentotal distance covered

= 50 + 100 = 150 m

and total time taken = 30 + 45 = 75 s

∴ Average speed =150 m

75 s = 2ms–1

Example 9.4 If an object moves with the speed of 10 ms–1 for 10 s and with8 ms–1 for 20 s, then total distance covered will be the sum of distance coveredin 10 s and the distance covered in 20 s = 10 × 10 + 8 × 20 = 260 m

∴ The average speed =total distance covered

total time taken

= ( )260 m 260 m

10+20 s 30 s=

= 8.66 ms–1

Example 9.5 If a body moves 50 m with the speed of 5 ms–1 and then 60 m withspeed of 6 ms-1, then total distance covered

= 50 + 60 = 110 m

and total time taken will be the sum of time taken for 50 m and 60 m = 20 s

Thus, average speed =total distance covered

total time taken

=110 m

20 s = 5.5 ms–1

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Example 9.6 If an object moves 30 m toward north in 10 s and then 40 m eastwardin next 10s, The displacement of the object will be OB

= 2 230 40+ = 900 1600+ = 2500

= 50 m

∴ The average velocity =total displacement covered

total time taken

=( )

50 m 50 m= = 2.5

10+10 s 20 s ms–1

Fig 9.16

Example 9.7 If an object moves along a circular track of radius 14 m and completeone round in 20 s then for one complete round total displacement is zero and theaverage velocity will also be zero.

From these examples you can conclude that:

(i) Instantaneous speed is the magnitude of instantaneous velocity but averagespeed is not the magnitude of average velocity.

(ii) Average velocity is less than or equal to the average speed.

(iii) Average velocity can be zero but not average speed.


1. Some of the quantities are given in column I. Their corresponding values arewritten in column II but not in same order. You have to match these valuescorresponding to the values given in column I:

Column I Column II

(a) 1 kmh–1 (i) 20 ms–1

(b) 18 kmh–1 (ii) 10 ms–1

(c) 72 kmh–1 (iii) 5/18 ms–1

(d) 36 kmh–1 (iv) 5 ms–1

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2. A cyclist moves along the path shown in the diagram and takes 20 minutes frompoint A to point B. Find the distance, displacement and speed of the cyclist.

Fig. 9.17

3. Identify the situation for which speed and average speed of the objects are equal.

(i) Freely falling ball

(ii) Second or minute needle of a clock

(iii) Motion of a ball on inclined plane

(iv) Train going from Delhi to Mumbai

(v) When object moves with uniform speed

4. The distance-time graph of the motion of an object is given. Find the averagespeed and maximum speed of the object during the motion.

Fig. 9.18

5. The distance travelled by an object at different times is given in the table below.Draw a distance-time graph and calculate the average speed of the object. Statewhether the motion of the object is uniform or non-uniform.

Table 9.7

Time (s) → 0 10 20 30 40 50

Distance (m) → 0 2 4 6 8 10

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6. A player completes his half of the race in 60 minutes and next half of the racein 40 minutes. If he covers a total distance of 1200 m, find his average speed.

7. A train has to cover a distance of 1200 km in 16 h. The first 800 km are coveredby the train in 10 h. What should be the speed of the train to cover the restof the distance? Also find the average speed of the train.

8. A bird flies from a tree A to the tree B with the speed of 40 km h–1 and returnsto tree A from tree B with the speed of 60 km h–1. What is the average speedof the bird during this journey?

9. Three players P, Q and R reach from point A to B in same time by followingthree paths shown in the Fig. 9.19. Which of the player has more speed, whichhas covered more distance?

Fig. 9.19

9.4. GRAPHICAL REPRESENTATION OF MOTION

It shows the change in one quantity corresponding to another quantity in the graphicalrepresentation.

9.4.1 Position-time Graph

It is easy to analyze and understand motion of an object if it is represented graphically.To draw graph of the motion of an object, its position at different times are shownon y-axis and time on x-axis. For example, positions of an object at different timesare given in Table 9.8.

Table 9.8 Position of different objects at different time

Time (s) 0 1 2 3 4 5 6 7 8 9 10

Position (m) 0 10 20 30 40 50 60 70 80 90 100

In order to plot position-time graph for data given in Table 9.8, we represent timeon horizontal axis and position on vertical axis drawn on a graph paper. Next, wechoose a suitable scale for this.

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For example, in Fig. 9.20 one division on horizontal axis represents 1 s of time intervaland one division on vertical axis represents in 10 m, respectively. If we join differentpoints representing corresponding position time data, we get straight line as shownin Fig. 9.20. This line represents the position-time graph of the motion correspondingto data given in Table 9.8.

Fig. 9.20 Position-time graph for the motion of a particle on the basis ofdata given in table

We note from the data that displacement of the object in 1st second, 2nd second,.....,10th second is the same i.e., 10 m. In 10 second, the displacement is 100 m.

Therefore, velocity is 100 m

10 s= 10 ms–1 for the whole course of motion. Velocity

during 1st second = 10 ms–1 and so on.

Thus, velocity is constant i.e., equal to 10 ms–1 throughout the motion. The motionof an object in which velocity is constant, is called uniform motion.

As you see Fig. 9.20, for uniform motion position-time graph is a straight line.

Like position-time graph, you can also plot displacement-time graph. Displacementis represented on the vertical axis and time interval on the horizontal axis. Sincedisplacement in each second is 10 m for data in table, the same graph (Fig. 9.20)also represents the displacement-time graph if vertical axis is labeled as displacement.

For good understanding you can observe the following graphs.

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(A) Uniform motion (B) Object is at rest

(C) Non-uniform motion, rate of change (D) Non-uniform motion, rate of changein position is increasing in position is decreasing

Fig. 9.21 Graph (A), (B), (C), (D)

9.4.2 Velocity-Time Graph

Take time on the horizontal axis and velocity on the vertical axis on a graph paper.Let one division on horizontal axis represent 1 s and one division on vertical axisrepresent 10 ms–1. Plotting the data in Table 9.9 gives us the graph as shown inFig. 9.22.

Table 9.9 Velocity-time data of objects A and B

Time (s) 0 1 2 3 4 5 6 7 8

Velocity of A (ms-1) 0 10 20 30 40 50 60 70 80

Velocity of B (ms-1) 0 10 10 10 10 10 10 10 10

Fig. 9.22 Velocity-time graph for the motion of object A and B on thebasis of data given in table

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Lines OR and PQ represent the motion of object A and B respectively. Thus, wesee that the velocity-time graph of motion represented in Table 9.9 is a straight lineand parallel to time axis for object B. This is so because the velocity is constantthroughout the motion. The motion is uniform. Consider the area under the graphin Fig. 9.22 for object B.

Area = (8s) × (10 ms–1) = 80 m. This is equal to the displacement of the objectB in 8 s.

Area under velocity-time graph = Displacement of the object during that timeinterval

Similarly for object A area under the graph in Fig. 9.22.

= ( ) ( )18 s 80 0

2× − ms–1

= ( ) ( )18 80 m

2× = 320 m

This is equal to the displacement of object A in 8 s.

Though, we obtained this result for object B for a simple case of uniform motion,it is general result.

Let x be displacement of an object in time t, moving with uniform velocity v, then

x = vt (for uniform motion)

You may have seen the motion of objects moving differently. Can you think whatmake this difference? Observe the motion of a ball on a floor. The ball slows downand finally comes to rest. This means that the velocity during different time intervalsof motion is different. In other words velocity is not constant. Such a motion is calledaccelerated motion.

9.5 ACCELERATION

In the previous section we have learnt about the non-uniform motion in which thechange in velocity in different intervals of motion is different. This change in velocitywith time is called acceleration. Thus, the acceleration of an object is defined asthe change in velocity divided by the time interval during which this occurs.

Acceleration =Change in velocity

Time interval

Its unit is ms–2. It is specified by direction. Its direction is along the direction of changein velocity. Suppose the velocity of an object changes from 10 ms–1 to 30 ms–1 ina time interval of 2 s.

Fig. 9.23 Changing velocity

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The acceleration, a =–1 –130 ms – 10 ms

2.0s = 10 ms–2

This means that the object accelerates in +x direction and its velocity increases ata rate of 10 ms–1 in every second.

If the acceleration of an object during its motion is constant, we say that object ismoving with uniform acceleration. The velocity-time graph of such a motion isstraight line inclined to the time axis as shown in Fig. 9.24.

For a given time interval, if the final velocity is more than the initial velocity, thenaccording to Fig. 9.24, the acceleration will be positive. However, if the final velocityis less than the initial velocity, the acceleration will be negative.

Fig. 9.24 velocity-time graph of an object moving with uniform acceleration

When velocity of the object is constant, acceleration will be zero. Thus, for uniformmotion, the acceleration is zero and for non-uniform motion, the acceleration is non-zero.

Example 9.8 Find the distance and displacement from the given velocity-time graphin Fig. 9.25.

Fig. 9.25

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Solution:Distance travelled = Area of ΔOAB + Area of ΔBCD

=1

2(25) × (20) +

1

2(10) × (20)

= 250 + 100 = 350 m

Displacement = Area of ΔOAB – Area of ΔBCD

=1

2(25) × (20) –

1

2 (10) × (20)

= 250 – 100 = 150 m

Example 9.9 From the given velocity-time graph obtain the acceleration-time graph.

Fig. 9.26

Solution: From the given graph acceleration for 0 – 10 s time interval

=15 0

10 0

−− = 1.5 ms–2

acceleration for 10 – 20s time interval in same as for 20 – 30s time interval

=20 15 5

30 10 20

− =− = 0.25 ms–2

acceleration for 30 – 40s time interval =20 20

40 30

−− = 0

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acceleration for 40 – 50 and 50 – 60s interval =30 20 10

60 40 20

− =− = 0.5 ms–2

For all the above time intervals the acceleration-time graph can be drawn as shownin Fig. 9.27.

Fig. 9.27


1. Describe the motion of an object shown in Fig. 9.28.

Fig. 9.28 Position-time graph of an object

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2. Compare the velocity of two objects where motion is shown in Fig. 9.29.

Fig. 9.29 Position-time graph for object A and B.

3. Draw the graph for the motion of object A and B on the basis of data given inTable 9.10.

Table 9.10

Time (s) 0 10 20 30 40 50

Position (m) for A 0 5 5 5 5 5

Position (m) for B 0 2 4 6 8 10

4. A car accelerates from rest uniformly and attains a maximum velocity of2 ms–1 in 5 seconds. In next 10 seconds it slows down uniformly and comesto rest at the end of 10th second. Draw a velocity-time graph for the motion.Calculate from the graph (i) acceleration, (ii) retardation, and (iii) distancetravelled.

5. A body moving with a constant speed of 10 ms–1 suddenly reverses its directionof motion at the 5th second and comes to rest in next 5 second. Draw a position-time graph of the motion to represent this situation.

9.6 EQUATIONS OF MOTION

Consider an object moving with uniform acceleration, a. Let u be the initial velocity(at time t = 0), v, velocity after time t and S, displacement during this time interval.There are certain relationships between these quantities. Let us find out.

We know that

Acceleration =Chnage in velocity

Time interval

∴ a =v u

t

−

or v = u + at ...(9.1)

Notes


217



This is called as the first equation of motion.

Also, we know that

Displacement = (average velocity) × (time interval)

or s =2

v ut

+⎛ ⎞⎜ ⎟⎝ ⎠

= 2

u at ut

+ +⎛ ⎞⎜ ⎟⎝ ⎠

( )v u at= +∵

or s = 21

2ut at+ ...(9.2)

This is called the second equation of motion.

If object starts from rest, u = 0 and

s = 210

2t at× +

or s = 21

2at

Thus, we see that the displacement of an object undergoing a constant accelerationis proportional to t2, while the displacement of an object with constant velocity (zeroacceleration) is proportional to t.

Now, if we take v u

at

−= and .2

v us t

+⎛ ⎞= ⎜ ⎟⎝ ⎠

and multiply them, we find that

a.s =( )

2

v u v ut

t

− +⎛ ⎞⎜ ⎟⎝ ⎠

= 2 2

2

v u−

or 2a.s = v2 – u2

or v2 = u2 + 2as ...(9.3)

This is called as third equation of motion. In case of motion under gravity ‘a’ canbe replaced by ‘g’.


1. A ball is thrown straight upwards with an initial velocity 19.6 ms–1. It was caughtat the same distance above the ground from which it was thrown:

(i) How high does the ball rise?

(ii) How long does the ball remain in air? (g = 9.8 ms–2)

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2. A brick is thrown vertically upwards with the velocity of 192.08 ms–1to thelabourer at the height of 9.8 m. What are its velocity and acceleration when itreaches the labourer?

3. A body starts its motion with a speed of 10 ms–1 and accelerates for 10 s with10 ms–2. What will be the distance covered by the body in 10 s?

4. A car starts from rest and covers a distance of 50 m in 10 s and 100 m in next10 s. What is the average speed of the car?

9.7 UNIFORM CIRCULAR MOTION

You may have seen the motion of the bicycle on a straight level road. Do all movableparts of the bicycle move alike? If not, then how are they moving differently? Doesthe peddling make a difference in these motions? Like Nimish, number of questionsyou may have in your mind. Let us try to answer these questions. Bicycle is movingon a straight road so its motion is rectilinear motion.

Fig. 9.30 Bicycle moving on a road

Now look at the wheels of the bicycle. Any point on the wheel of the bicycle alwaysremains at a constant distance from the axis of the wheel and moves around the fixedpoint i.e., axis of the wheel. On the basis of this description of motion of the wheelyou can decide very obviously that this motion is circular motion.

Similarly, can you think about the motion of the flywheel of the bicycle? During non-peddling, there is no circular motion of flywheel and it moves in a straight line thus,its motion is rectilinear motion. But during the peddling its motion is circular motioncan you think about the motion of any part of the bicycle which has two types ofmotion at the same time? Yes, during the circular motion of the wheel or flywheel,they are also advancing in forward direction on a straight road. Thus, there motionis circular motion as well as rectilinear motion at the same time.

Now consider the motion of an object along a circular track of radius R throughfour points A, B, C and D on the track as shown in Fig. 9.31. If object completeseach round of motion in same time, than it covers equal distance in equal intervalof time and its motion will be uniform motion. Since during this uniform motion equaldistance is being covered in equal interval of time, therefore, the ratio of distance

Notes


219



Fig. 9.31 Circular motion

covered to the time taken i.e., speed will remain constant. It means in uniformcircular motion speed remains constant.

Now think about velocity, velocity remainsalong the direction of motion. In Fig. 9.31you can see the direction of motion changesat every point as shown at point A, B, C andD. Since there is a change in direction ofmotion, therefore, the direction of velocityalso changes. We can say that in uniformcircular motion, velocity changes due tochange in direction of motion and the motionof the object is accelerated motion. Thisacceleration is due to change in the directionof motion. But in this motion speed remains constant. How interested this motionis because a body moving with constant speed acquires acceleration.

Think and Do

K I L O M E T R E T O

S P E E D T O N C N E

O N D I S T A A N O E

P D I S P L A C D I A

A N S V E L O C I T Y

T A P P E E R C S A N

K A L U D I N E T R A

T E A M Y O Y L A E D

M A C H I N E E N L L

E P E P T A D R C E K

T O M F T R E A E C D

R N E N G I N T G C Q

E E N K L O M E T A R

In the above word grid identify the meaningful words, related to description of motion,in horizontal or vertical columns in sequence and define them (at least three).

Notes



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220


1. In circular motion the point around which body moves

(a) always remain in rest

(b) always remain in motion

(c) may or may not be in motion

(d) remain in oscillatory motion

2. In uniform circular motion

(a) speed remain constant

(b) velocity remain constant

(c) speed and velocity both remain constant

(d) neither speed nor velocity remain constant

3. A point on a blade of a ceiling fan has

(a) always uniform circular motion

(b) always uniformly accelerated circular motion

(c) may be uniform or non-uniform circular motion

(d) variable accelerated circular motion

WHAT YOU HAVE LEARNT

� If a body stays at the same position with time, it is at rest.

� If the body changes its position with time, it is in motion.

� Motion is said to be rectilinear if the body moves in the same straight line allthe time. e.g., a car moving in straight line on a level road.

� The motion is said to be circular if the body moves on a circular path; e.g. themotion of the tip of the second needle of a watch.

� The total path length covered by a moving body is the distance travelled by it.

� The distance between the final and initial position of a body is called itsdisplacement.

� Distance travelled in unit time is called speed, whereas, displacement per unittime is called velocity.

Notes


221



� Position-time graph of a body moving in a straight line with constant speed isa straight line sloping with time axis. The slope of the line gives the velocity ofthe object in motion.

� Velocity-time graph of a body in straight line with constant speed is a straightline parallel to time axis. Area under the graph gives distance travelled.

� Velocity-time graph of a body in straight line with constant acceleration is astraight line sloping with the time axis. The slop of the line gives acceleration.

� For uniformly accelerated motion

v = u + at

s = ut + 1

2at2

and v2 = u2 + 2as

where u = initial velocity, v = final velocity, and s = distance travelled in t seconds

TERMINAL EXERCISE

1. An object initially at rest moves for t seconds with a constant acceleration a.The average speed of the object during this time interval is

(a)2

a t⋅; (b) 2a t⋅ ; (c)

21

2a t⋅ ; (d)

21

2a t⋅

2. A car starts from rest with a uniform acceleration of 4 ms–2. The distance travelledin metres at the ends of 1s, 2s, 3s and 4s are respectively,

(a) 4, 8, 16, 32 (b) 2, 8, 18, 32

(c) 2, 6, 10, 14 (d) 4, 16, 32, 64

3. Does the direction of velocity decide the direction of acceleration?

4. Establish the relation between acceleration and distance travelled by the body

5. Explain whether or not the following particles have acceleration:

(i) a particle moving in a straight line with constant speed, and

(ii) a particle moving on a curve with constant speed.

6. Consider the following combination of signs for velocity and acceleration of anobject with respect to a one dimensional motion along x-axis and give examplefrom real life situation for each case:

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Table 9.11

Velocity Acceleration Example

(a) Positive Positive Ball rolling down on a slopelike slide or ramp

(b) Positive Negative

(c) Positive Zero

(d) Negative Positive

(e) Negative Negative

(f) Negative Zero

(g) zero Positive

(h) Zero Negative

7. A car travelling initially at 7 ms–1 accelerates at the rate of 8.0 ms–2 for an intervalof 2.0 s. What is its velocity at the end of the 2 s?

8. A car travelling in a straight line has a velocity of 5.0 ms–1 at some instant. After4.0 s, its velocity is 8.0 ms–1. What is its average acceleration in this time interval?

9. The velocity-time graph for an object moving along a straight line has shown inFig. 3.32. Find the average acceleration of this object during the time interval0 to 5.0 s, 5.0 s to 15.0 s and 0 to 20.0 s.

Fig. 9.32

10. The velocity of an automobile changes over a period of 8 s as shown in the tablegiven below:

Notes


223



Table 9.12

Time (s) Velocity (ms–1) Time (s) Velocity (ms–1)

0.0 0.0 5.0 20.0

1.0 4.0 6.0 20.0

2.0 8.0 7.0 20.0

3.0 12.0 8.0 20.0

4.0 16.0

(i) Plot the velocity-time graph of motion.

(ii) Determine the distance the car travels during the first 2 s.

(iii) What distance does the car travel during the first 4 s?

(iv) What distance does the car travel during the entire 8 s?

(v) Find the slope of the line between t = 5.0 s and t = 7.0 s. What doesthe slope indicate?

(vi) Find the slope of the line between t = 0 s to t = 4 s. What does this sloperepresent?

11. The position-time data of a car is given in the table given below:

Table 9.13

Time (s) Position (m) Time (s) Position (m)

0 0 25 150

5 100 30 112.5

10 200 35 75

15 200 40 37.5

20 200 45 0

(i) Plot the position-time graph of the car.

(ii) Calculate average velocity of the car during first 10 seconds.

(iii) Calculate the average velocity between t = 10 s to t = 20 s.

(iv) Calculate the average velocity between t = 20 s and t = 25 s. What canyou say about the direction of the motion of car?

12. An object is dropped from the height of 19.6 m. Draw the displacement-timegraph for time when object reach the ground. Also find velocity of the objectwhen it touches the ground.

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224

13. An object is dropped from the height of 19.6 m. Find the distance travelled byobject in last second of its journey.

14. Show that for a uniformly accelerated motion starting from velocity u andacquiring velocity v has average velocity equal to arithmetic mean of the initial(u) and final velocity (v).

15. Find the distance, average speed, displacement, average velocity and accelerationof the object whose motion is shown in the graph (Fig. 9.33).

Fig. 9.33

16. A body accelerates from rest and attains a velocity of 10 ms-1 in 5 s. What isits acceleration?

ANSWERS TO INTEXT QUESTIONS

9.1

1. (c) 2. (a) 3. (b) 4. (a) 5. (c)

6.

Fig. 9.34

Notes


225



9.2

1. (a) (iii) (b) (iv) (c) (i) (d) (ii)

2. Distance = 140 m, Displacement = 100 m, Speed = 7 ms–1

3. When object moves with uniform speed

4. 2ms–1, 5 ms–1

5. Average speed = 0.2 ms–1, motion is uniform motion

Fig. 9.35

6. 0.2 ms–1 7. 63 km h–1 8. 48 km h–1 9. R, R

9.3

1. For first five seconds object moves with constant speed i.e. 2ms–1. From 5 to15 second it remains at rest and then from 15 to 20 seconds it moves withconstant speed 2 ms–1.

The motion of the object is not uniform.

2. Velocity of object A is 4 times the velocity of B.

3.

Fig. 9.36

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226

4.

Fig. 9.37

(i) a = 0.4 ms–2, (ii) –a = 0.4 ms–2, (iii) 10 m

5.

Fig. 9.38

9.4

1. (i) 19.6 m, (ii) 4 s 2. Zero and 9.8 ms–2

3. 600 m 4. 7.5 ms–1

9.5

1. (a) 2. (a) 3. (b)

Notes


227

Force and Motion


10

FORCE AND MOTION

In the previous lesson you have learnt about the motion of a body along a straightline. You also know that motion can be uniform or non-uniform. You might have seenthat a body at rest can be brought to motion and a moving body can be broughtto rest. Do you know what makes bodies at rest to move or stop if they are in motion?What changes the speed or direction of a moving object? Why do the dust particlesget detached from a carpet when it is beaten with a stick? Why does a ball rollingalong the ground stops after moving through some distance? Why cutting tools alwayshave sharp edges?

In this lesson we shall try to find the answer of all such questions.

OBJECTIVES

After completing this lesson, you will be able to:

� explain the cause of motion - concept of force;

� distinguish between balanced and unbalanced forces;

� define the terms inertia, mass and momentum;

� state and explain the three laws of motion and explain their significancein daily life and nature;

� derive a relationship between force, mass and acceleration;

� explain the force of friction and analyze the factors on which it depends;

� illustrate and appreciate that rolling friction is less than sliding friction;

� cite examples from everyday life where importance of friction can beappreciated and

� explain the terms thrust and pressure, citing example from daily lifesituations.

Notes


MODULE - 3 Force and Motion

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228

10.1 FORCE AND MOTION

If we place a ball on a flat surface, it will remain there until unless we disturb it. Itwill move only when either we push it or pull it. This push or pull acting on an objectis known as a force. What else happens when we apply force on an object? Think!Let us do an activity to understand it.

ACTIVITY 10.1

Hold an inflated balloon between your palms. Now, apply a force on it by pressingyour palms (Fig. 10.1). What do you observe?

You will observe that on pressing the balloon,its shape changes. Thus, we can say that onapplying force, the shape of a body can bechanged. Can you now think of some othereffect of force?

While playing football if you want to changethe direction of the moving ball you will haveto kick the ball in a particular direction. Whenyou kick the ball, you apply certain force tochange the direction of the moving ball.Similarly, you can also change the speed ofa moving object by applying force on it. Forexample the speed of a moving bicycle canbe changed by applying brakes on it.

Thus, on the basis of above examples and activities we can say that the force appliedon an object can

� make the object move from rest.

� change the speed of a moving object.

� change the direction of motion of the object

� change the shape of the object.

Now, it is time to assess how much have you learnt?


1. Is there any force applied when a cricket player changes the direction of ballby using his/her bat?

Fig. 10.1 Shape of balloon changes onapplying force on it

Notes


229

Force and Motion


2. Give an example from your daily life in which the shape of an object changesby applying a force.

10.2 BALANCED AND UNBALANCED FORCES

Have you even seen a game of tug-of-war (Fig. 10.2)? In this game when the twoteams pull with equal force they apply balanced forces on the rope. The rope thusremains stationary. When one of the teams applies greater force, it is able to pullthe other team and the rope towards their side. In this case forces are unbalanced.

Fig. 10.2 Tug of war

For understanding the concepts of balanced and unbalanced forces, let us performthe following activity.

ACTIVITY 10.2

Place a brick on a table. Push the brick towards left with your right hand. Whatdo you observe? The brick begins to move to the left direction [Fig. 10.3 (a)]. Nowpush the brick towards right with your left hand. In which direction the brick movesthis time [Fig. 10.3 (b)]?

Fig. 10.3 Unbalanced and balanced forces

(c)

(a) (b)

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230

Now push the brick from both the sides with equal forces [Fig. 10.2 (c)]. What doyou observe? In this case you will observe that the brick does not move in anydirection. Can you think why the brick does not move this time? In fact, in this casethe two forces balance each other. Such forces are called balanced forces.

What type of changes can be produced by balanced forces? As seen above, balancedforces do not change the state of rest or motion of the object on which they areapplied. Now recall the activity 10.1 and think whether it was balanced or unbalancedforce on the balloon? Yes, you are right, it was the balanced force applied by yourpalms that changed the shape of balloon.

What happen when the two opposite forces acting on the brick are of differentmagnitudes? In this case the brick would begin to move in the direction of greaterforce. Such forces are called unbalanced forces. Unbalanced forces acting on anobject may change its state of rest or motion.

Try to find out some more examples of balanced and unbalanced forces.


1. What are balanced forces?

2. Can a balanced force produces any acceleration in a body?

3. What type of change can be produced by an unbalanced force in a body?

10.3 NEWTON’S LAWS OF MOTION

10.3.1 Inertia

You would have seen that whenever we shake the branches of a tree vigorously,the leaves and fruits get detached. Similarly, when you beat a carpet with a stick,you will see that the dust particles get detached from the carpet. Do you know why?

The answer to all such questions is inertia. What is inertia? We can understand theproperty of inertia by doing a simple activity.

ACTIVITY 10.3

Take a smooth sheet of paper (30 cm × 8 cm) and place it on a table with somepart of it coming out of the edge of the table. Now place a glass half filled with wateron the paper. Remove the paper with a jerk (Fig. 10.4). What do you observe?You will find that the glass remains in its position. The inertia of the glass preventsit from moving with the paper.

Notes


231

Force and Motion


Fig. 10.4 Glass remains in its position due to inertia

Thus we can say that the inertia is the tendency of objects to stay at rest or to keepmoving with the same velocity. You can find out some more examples of inertia fromyour daily life. In fact it is the inertia due to which a sprinter keeps running for sometime even after crossing the finish line. Similarly, you would have noticed that it isdifficult to take out the tomato sauce from a bottle by just inverting it. However, itis easy to take out the sauce from the bottle by giving a sudden jerk to it. By movingthe bottle in the downward direction the sauce comes in motion. When the bottlestops suddenly, the sauce remains in motion due to inertia of motion and comes outof the bottle.

10.3.2 Inertia and Mass

By now you have learnt that due to inertia an object offer resistance to change itsstate of motion. Do all objects have the same inertia? Let us find out.

Push an empty box on a smooth surface. Now try to push a similar box full of bookson the same surface. What do you find? Why is it easier to push an empty box thana box full of books?

Now suppose you are asked to stop a table tennis ball and a cricket ball movingwith the same velocity. On which ball you are supposed to apply more force to stopit. You will find that cricket ball require more force to stop as compared to tabletennis ball.

Thus all objects do not resist a change in their state of rest or motion equally. Massiveobjects resist more than lighter ones. What do you conclude from these observations?We can say that mass is a measure of inertia.

10.3.3 Newton’s First Law of Motion

You have learnt that an object offer resistance to change in its state of motion. Thiswas studied by Newton in detail and he presented his findings in the form of three

Notes



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232

fundamental laws that govern the motion of objects. Newton’s first law of motionis stated as follows:

“Every body continues in its state of rest or of uniform motion in a straight line untilunless it is compelled by some unbalanced force to change that state.”

Newton’s first law of motion tells us that all bodies resist a change in their state ofmotion. We know that this property of bodies is called inertia. That is why, Newton’sfirst law of motion is also known as the law of inertia.

First law of motion has many applications in our daily life. Why do the passengersstanding in a bus fall in the backward direction when the stationary bus begins tomove suddenly (Fig. 10.5)?

Fig. 10.5 Passengers falling in the backward direction when the bus starts suddenly

This observation can be explained on the basis of first law of motion. The feet ofpassengers are in contact with the bus. When the bus starts suddenly, the feet startmoving with the bus. But the upper part of the passengers tries to remain at restdue to inertia and tends to fall in the backward direction.

What happen when the moving bus stops suddenly? In this case the passengersstanding in the bus fall in the forward direction. Can you think the reason of it onthe basis of the explanation of the above example?

Fig. 10.6 Passengers falling forward as the moving bus stops suddenly

Notes


233

Force and Motion


Now you should be able to explain why do the dust particles get detached froma carpet when it is beaten with a stick? Try to explain it on the basis of first lawof motion.

10.3.4 Momentum

You have learnt in the earlier section that the force required to stop a moving bodydepends upon its mass. Now suppose two balls of same mass are moving withdifferent velocities. Which ball will need more force to stop? You will find that thefaster moving ball require more force to stop it. Thus, the force required to stop abody also depends upon its velocity.

You must have noticed that a small bullet when fired from a gun can kill a person.But the same bullet if thrown with hand can hardly do any harm. Similarly a truckparked along a road side does not require any attention. But a moving truck maykill a person standing in its path. Is it only the velocity of the truck which makesus frightened? If it is so, then a toy car moving with the same velocity as the truckwould have equally frightened to us.

From these observations it appears that the impact produced by the objects dependson their mass and velocity. These two quantities help us to define a new quantitycalled momentum.

The momentum, p of a moving body is defined as the product of its mass, m andvelocity, v. That is

p = mv (10.1)SI unit of momentum is kilogram-metre per second (kg m s–1). Momentum has bothmagnitude and direction. Its direction is same as that of velocity.

10.3.5 Newton’s second law of motion

According to Newton’s first law of motion the application of an unbalanced forcebrings a change in the velocity of an object. Thus, the force can produce a changeof momentum. Newton’s second law of motion establishes a relationship betweenforce and change in momentum.

Second law of motion states that the rate of change ofmomentum of a body is directly proportional to theforce acting on it and takes place in the samedirection as the force.

Newton’s second law of motion also gives a relationbetween force and acceleration. Let us derive thisrelationship.

Suppose the velocity of an object of mass m changes fromu to v in time t by the application of a constant force F.

Sir Isaac Newton(1642-1727)

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234

The magnitude of initial and final momentum of the object will be p1 = mu andp2 = mv respectively. The change in momentum in time t = p2 – p1.

The rate of change of momentum = ( )2 1p p

t

−

According to second law of motion, the magnitude of the force F, is

F ∝ 2 1p p

t

−

or F =( )2 1k p p

t

−...(10.2)

where k is constant of proportionality.

Substituting the value of p1 = mu and p2 = mv, we get

F =( )k mv mu

t

−

=( )km v u

t

−

Now, v u

t

− is the rate of change of velocity, which is the acceleration ‘a’. Therefore,

we have

F = kma (10.3)

We choose the unit of force in such a manner that the value of k becomes one. Forthis we can define one unit of force as that amount which produces an accelerationof 1 m/s2 in an object of 1kg mass. So that:

1 unit of force = k (1 kg) × (1 ms–2)

Thus, the value of constant k becomes 1. Therefore, from equation (10.3)

F = ma (10.4)

The unit of force is called newton and its symbol is N.

So a force of 1 newton will produce an acceleration of 1 m/s2 on an object of mass1 kg.

Can you estimate, how much is 1 N force?

For this, let us experience it. Keep a mass of 100 g on your palm. How much forceyou feel on your palm? Calculate this force.

Notes


235

Force and Motion


From equation 10.4,

F = ma

Here, m =1

10 kg and a = 10 ms–2 (approximately)

Therefore, F =1

10kg × 10 ms–2 = 1 N

Thus the force exerted by a mass of 100 g on your palm is approximately equalto 1 newton.

10.3.6 Some Example of Second Law of Motion from Daily Life

In our everyday life we see many applications of second law of motion. In manysituations we try to decrease or increase the rate of change of momentum by changingthe time in which the change of momentum takes place. Let us consider someexamples.

(a) While catching a fast moving cricket ball, why does a fielder moves his handsbackward?

By doing so the fielder increases the time duration in which the momentum of theball becomes zero (Fig. 10.7). As the rate of change of momentum decreases, a smallforce is required for holding the catch. So the hands of the fielder do not get hurt.

Fig. 10.7 A fielder moves his hands backward while holding a catch

(b) Why does a person get hurt when he falls on a cemented floor?

Just before touching the floor, the person has some initial velocity, say u, whichbecomes zero when he comes to rest. Thus the momentum of the person becomeszero within a very short time. As the rate of change of momentum is very high, sovery large force is exerted on the person, thereby hurting him. On the other hand,

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Moving Things

236

if he falls on sand or husk or on a foam mattresses, he does not get hurt due tolonger period of time in making momentum zero and hence reduction of force.

(c) How does a karate player breaks a pile of tiles or a slab of ice with a singleblow?

The karate player hits the pile of tiles or a slab of ice as fast as possible with herhand. In doing so the entire momentum of the hand is reduced to zero in a very shorttime. As a result, the force delivered on the tiles or slab of ice is large enough tobreak it.

(d) You would have noticed that when a bundle tied with a string is lifted quicklyby holding it, the string breaks (Fig. 10.8). Can you now explain why the stringbreaks in this case?

Fig. 10.8 The string breaks when the bundle is lifted quickly.

Example 10.1: What is the acceleration produced by a force of 15 N exerted onan object of mass 3 kg?

Solution: According to second law of motion

F = ma

Here m = 3 kg and F = 15 N

Therefore, 15 N = 3 kg × a

or a =15 N

3 kg = 5 ms–2

Notes


237

Force and Motion


Example 10.2: What force accelerates a 50 kg mass at 5 ms–2?

Solution: Newton’s second law gives

F = ma

Here, m = 50 kg and a = 5 ms–2

Therefore, F = 50 kg × 5 ms–2

= 250 N

10.3.7 Newton’s Third Law of Motion

You must have noticed that when a rubber balloon filled with air is released, theballoon moves opposite to the direction of the air coming out of it (Fig. 10.9). Whydoes the balloon move in a direction opposite tothe direction in which the air escapes? Let us findout.

You must have also noticed that when you jumpfrom a boat to the river bank, the boat moves inthe backward direction (Fig. 10.10). Why doesthis happen?

While jumping out of the boat, your foot exertsa backward force on the boat. This force is calledaction. At the same time a force is exerted bythe boat on your foot, which makes you moveforward. This force is known as reaction.Remember that two bodies and two forces areinvolved in this problem. You pushed the boatbackward and the boat pushes you forward.These two forces are equal in magnitude butopposite in direction.

Fig. 10.10 A girl jumping out of a boat

Fig. 10.9 A balloon movesopposite to the direction in

which air escapes

Notes



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238

Let us consider the balloon problem again. In this case the air coming out of theballoon (action) exerts a force of reaction on the balloon and this force pushes theballoon backwards (reaction).

Newton in his third law of motion stated a relation between action and reaction. Ac-cording to this law, to every action there is an equal and opposite reaction. Theaction and reaction act on two different bodies if action and reaction are on samebody they will constitute a balanced force and body will not move.

Look at the Fig. 10.11 and find out the action and reaction forces and try to analysewheather the truck will move or not.

Fig. 10.11

There are three significant features of third law of motion:

(i) We cannot say which force out of the two forces is the force of action and whichone is the force of reaction. They are interchangeable.

(ii) Action and reaction always act on two different bodies.

(iii) The force of reaction appears so long as the force of action acts. Therefore,these two forces are simultaneous.

Remember, it is not necessary that the twobodies, amongst which the forces of actionand reaction act are in contact. They may bequite far from each other. For example,attraction or repulsion between two magnetscan take place even without being in contact(Fig. 10.12).

Fig. 10.12 Repulsion between twomagnets

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Force and Motion


Do you know that action and reaction forces enable us to walk on the surface ofthe earth? Let us see how? While walking on the ground we push the ground withour foot in the backward direction. This is the force of action.

In return the ground exerts an equal force of reaction on our foot in the forwarddirection. The force that actually makes us walk in the forward direction is thisreaction force.

Similarly, during swimming we push the water in the backward direction, with ourhands and feet, to move in forward direction. It is the reaction to this force that pushesus forward (Fig. 10.13).

Fig. 10.13 A swimmer pushes the water backwards with hands to move inforward direction.

It may be interesting for you to know that rocketsand jet-planes also work on the principle of actionand reaction. In each of these, when the fuel burns,hot burning gases are ejected from the tail. The hotgases come out in the backward direction and therocket or the jet plane moves in the forward direction(Fig. 10.14).

Now think, why a rifle kicks backward when we firea bullet?

10.3.8 Conservation of Momentum

Law of conservation of momentum is a very importantlaw of science. According to this law, if two or moreobjects collide with each other, their total momentumremains conserved before and after the collisionprovided there is no external force acting on them.

Fig. 10.14 Working of jetplanes and rockets

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From the Newton’s laws of motion, we know that the rate of change of momentumis equal to the force.

If p1 = initial momentum and p2 = final momentum after time t, then

F = 2 1p p

t

−

Now, if F = 0, then we have p1 = p2. Which shows that the momentum of a systemremains unchanged (or conserved) if no force is acting on it?

You can verify the law of conservation of momentum with the help of a simple activity.

ACTIVITY 10.4

Take a plastic channel of about 40 cm length and seven marbles of same size. Placethe channel on a horizontal table and put the marbles on the channel touching eachother as shown in figure 10.15. Remove one marble and keep it at a distance ofabout 15 cm from the rest. Hit this marble with your fore finger gently so that it collideswith other marbles. What do you observed?

Fig. 10.15 Arrangement to show the law of conservation of momentum

You will find that after the collision, the moving marble comes to rest and the lastmarble out of the rest moves ahead. Try to guess the speed of this marble after thecollision and compare it with the speed of marble you had thrown before the collision.Do the two speeds appear to be equal? What does it indicate? If the speeds areequal then the total momentum of the marble is same before and after the collision.

Repeat this activity by removing two marbles and striking them with the five marblesat rest. What do you observe this time? What conclusion do you derive from thisactivity? You will find that in each case, total momentum of marbles before collisionis same as after collision.

Have you ever seen a toy as shown here?If not, try to find this toy in a toy shop ora science museum. Can you tell the principleon which this toy works?

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Force and Motion


Example 10.3: A bullet of mass 0.03 kg is fired with a velocity of 100 ms–1 froma rifle of mass 3 kg. Calculate the recoil velocity of the rifle.

Solution:

Here, mass of the rifle m1 = 3 kg

mass of the bullet m2 = 0.03 kg

Initial velocity of the riffle u1 = 0

Initial velocity of the bullet u2 = 0

Final velocity of the rifle = v1 (say)

Final velocity of the bullet v2 = 100 ms–1

According to the law of conservation of momentum,

m1u1 + m2u2 = m1v1 + m2v2

On substituting the given values,

0 + 0 = 3 × v1 + (0.03) × 100

v1 =100 0.03

3

− × = –1.0 ms–1

∴ Recoil velocity of the rifle = –1.0 ms–1

Negative sign indicates that the rifle would move in the direction opposite to thatbullet.

Example 10.4: A rifle having a mass of 5 kg fires a bullet at a speed of 250ms–1. If the rifle recoil with a velocity of 1 ms–1 then find the mass of the bullet.

Solution:

Here, M = 5 kg; m = ?

V = –1 ms–1; v = 250 ms–1

U = 0 u = 0

According to the law of conservation of momentum

MU + mu = MV + mv

0 = MV + mv

m =MV

v

− =

( )5 1

250

− × − =

1

50 = 0.02 kg

So, Mass of the bullet = 0.02 kg or 20 g

Negative sign indicates that the rifle would move in the direction opposite to thatobullet.

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242


1. Why does water comes out from a wet piece of cloth when you shake it?

2. Why do we fall forward, when a moving bus stops suddenly?

3. Two similar trucks are moving on a road with the same velocity. One of themis empty while the other one is loaded. Which of the two has more momentum?

4. If a body of mass 5 kg moves with a velocity of 10 ms–1, then what is themomentum of the body?

5. Why does a boxer move his head backward while taking an oncoming punch?

10.4 FRICTION

You might have noticed that a ball rolling along the ground stops after moving throughsome distance. Similarly a moving car begins to slow down the instant its engine isswitched off and finally it stops. Why does it happen? Let us find out.

10.4.1 Force of Friction

According to Newton’s first law of motion, a moving body continues to move alonga straight line until unless an external force is applied on it. Is this external force slowsdown the motion of the ball or the car? Think! Infact the ball or car is slowed downby a force called friction. Friction exists between the surfaces of all materials whichare in contact with each other. The direction of the frictional force is always in adirection opposite to the motion.

Now, try to analyze the forces acting on an object moving with a constant velocity.If an object is to move with a constant velocity, a force equal to the opposing forceof friction must be applied. In that condition the two forces are balanced forces. Theyexactly cancel one another and the net force on the body is zero. Hence theacceleration produced in the body is zero and the body maintains its velocity. It neitherspeeds up nor slows down.

The resistive force, before the body starts moving on a surface is called staticfriction. Once a body starts moving on a surface the friction between them is calledsliding or kinetic friction. You should remember that the sliding friction is slightlyless than the static friction.

10.4.2 Factors affecting friction

You must have seen that it is easier to move a bicycle on a concrete road than ona rough road. Why is it so? Does friction depend upon the smoothen or the roughnessof the surfaces? Let us find out.

Notes


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Force and Motion


ACTIVITY 10.5

Set up an inclined plane on a table as shown in Fig. 10.16. Mark a line near thetop edge of the inclined plane. Now hold a pencil cell on this line. Release the pencilcell. What do you observe? The cell moves down the inclined plane and continuesto move for some distance on the table. Note down the distance upto which thecell move on the table.

Fig. 10.16 Pencil cell covers different distances on different type of surfaces

Now place a glass sheet on the table. Again release the pencil cell from the line onthe inclined plane and note the distance up to which the cell moves on the glass plate.Repeat this activity by spreading a uniform layer of sand on the table.

In which case the distance covered by the pencil cell is maximum? In which caseit is minimum? What do you conclude from this activity?

You will find that the distance moved by the cell is maximum on the glass surfaceand minimum over the sand. This difference is due to the friction offered by differenttype of surfaces. Smooth glass surface offers less friction compared to a rough sandbed. Thus smoothness of the surfaces is one of the factor on which frictiondepends.

You might have observed that more force is needed to move a heavy box than tomove a lighter box on the same surface. It is so because the heavy box has graternormal reaction (reaction of the surface on the box against the action of its weight)and hence greater frictional force. Thus friction also depends upon the normalreaction.

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244

10.4.3 Advantages and disadvantages of friction

The friction plays a very important role in our day to day life. It has several advantagesas well as disadvantages.

(a) Advantages of friction

Have you ever walked on ice or a wet marble floor? You might have found that itdifficult to balance your body. The force of friction developed between the soles ofyour shoes and the ground helps us to move. Had there been no friction, walkingor running would have been impossible.

You can write with a pen on page or with a chalk on the blackboard due to friction.Buildings may be constructed only due to force of friction between different buildingmaterials. Without friction, you could not fix a nail on the wall.

Tyres of automobiles are treaded to increase the friction between tyres and surfaceof the road. Thus the tyres get better grip with the ground. The breaks applied inautomobiles also work only due to friction.

Can you think of some more examples from your daily life where friction is useful?

(b) Disadvantages of friction

Due to friction, a lot of energy is wasted in the form of heat that causes wear andtear of the moving parts of a machine. Friction also reduces efficiency of the machinesas considerable amount of energy is wasted in overcoming friction. However, theefficiency of a machine can be increased by putting a suitable lubricant between itsmoving parts.

In most of the machines, to reduce friction ball bearings are used between the movingparts. By using the ball bearing the sliding friction is replaced by rolling friction.As the rolling friction is less than the sliding friction, therefore, the friction betweenthe moving parts is reduced.

Friction also wears out the soles of shoes. You would have seen that the steps offoot over-bridge at railway stations also wear out due to friction.

Vandana and Navneet are racing on rock ice with the specially designed shoes shownin the Fig. A and B respectively. Who will win?

(A) Shoes for Vandana (B) Shoes for Navneet

Cite some more example from you daily life where friction is undesirable.

Notes


245

Force and Motion



1. Why does a fast moving car slow down when its engine is switched off?

2. Why do we slip when we step on a banana peel?

3. Why are tyres of automobiles treaded?

10.5 THRUST AND PRESSURE

Observe some bodies around you like table, desk, bucket full of water, etc. Theypress the floor with a force equal to their own weight. You know that weight is theforce acting vertically downwards. As the surface of the floor can be taken ashorizontal, therefore, the force with which each of the above mentioned bodiespresses the floor is directed perpendicular to the surface of the floor. The force actingupon the surface of a body perpendicular to it is called thrust.

Let us find out the effect of thrust acting on a surface.

ACTIVITY 10.6

Take a small wooden board (10 cm × 10 cm × 1.0 cm) with four nail fixed at eachcorner as shown in Fig. 10.17 (a). Fill a tray with sand to a depth of about 6 cm.Place the wooden board on sand with the nail-heads downwards [Fig. 10.17 (b)]Also put about 500 g weight on the board. Observe the depth of the nails upto whichthey penetrate into the sand.

(a) (b) (c)

Fig. 10.17 (a), (b), (c) Arrangement to show that pressure depends uponthe area on which force is exerted

Now place the wooden board on the sand with pointed side of the nails facingdownwards and put the same weight on the board as in the previous case [Fig. 10.17(c)]. Again observe the depth of the nails upto which they penetrate upto the sand.

In which of the above two cases the penetration is more? You will find that thepenetration is more in the second case.

Thus the action of the given thrust depends on the area of the surface it acts upon.The smaller the area on which the thrust acts, the more evident is the result of itsaction. The thrust on unit area is called pressure. Thus

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246

Pressure =thrust

area...(10.5)

The SI unit of pressure is Nm–2. This unit has also been given a specific name pascal(Pa) in honour of the scientist named Blaise Pascal.

Pascal was a French philosopher andmathematician. He formulated the famous Pascal’slaw of hydraulics regarding transmission ofpressure through fluids. He also invented one ofthe earliest calculating machines. The unit ofpressure pascal (Pa) was named in his honour.

Equation (10.5) shows that the same force acting on a smaller area exerts a largerpressure and a smaller pressure on a larger area. This is the reason why cutting toolslike knives and axes always have sharp edges.

In many cases it is desirable to decrease pressure. In such cases the area on whichthe thrust is acting should be increased. For example, foundation of buildings anddams are made on larger area. Similarly trucks and vehicles used to carry heavy loadshave much wider tyres. Also army tank weighing more than a thousand tonne restsupon a continuous chain.


1. Why does a porter carrying a heavy load place a round piece of cloth on hishead?

2. Why a nail has a pointed tip?

3. Why shoulder bags are provided with broad straps?

4. State the SI unit of pressure.


� Unbalanced forces acting on an object may change its state of rest or motion.

� Balanced forces do not change the state of rest or motion of an object. Balancedforces can change the shape of the object on which they are applied.

� Inertia is the tendency of objects to stay at rest or to resist a change in theirstate of motion.

Blaise Pascal (1623-1662)

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Force and Motion


� The mass of an object is a measure of its inertia.

� Newton’s first law of motion states that, everybody continues in its state of restor of uniform motion in a straight line until unless it is compelled by someunbalanced force to change that state.

� The momentum of a body is the product of its mass and velocity. The SI unitof momentum is kg ms–1.

� Second law of motion states that the rate of change of momentum of a bodyis directly proportional to the force acting on it and takes place in the samedirection as the force.

� The unit of force is newton and its symbol is N. A force of 1 newton will proceedon acceleration of 1 ms–2 on an object of mass 1 kg.

� Newton’s third law of motion states that, to every action there is always an equaland opposite reaction. Action and reaction always act on two different bodies.

� According to the law of conservation of momentum, in an isolated system thetotal momentum remains conserved.

� Force of friction always opposes motion of bodies. Friction depends on thesmoothness of surfaces in contact. It also depends upon the normal reaction.

� Rolling friction is less than the sliding friction.

� Force acting perpendicular to the surface of a body is called thrust.

� Thrust per unit area is called pressure. The SI unit of pressure is Nm–2. Thisunit is known as pascal (Pa).

TERMINAL EXERCISE

1. Why does a sprinter keep running for sometime even after crossing the finishline?

2. Why is it advised to tie the luggage with a rope on the roof of busses?

3. Why do the dust particles from the hanging blanket fall off when it is beatenwith a stick?

4. State Newton’s first law of motion. Why do the passengers standing in astationary bus fall in the backward direction when the bus begins to movesuddenly.

5. Define momentum. How the rate of change of momentum is related to force?

6. If a body of mass 10 kg moves with a velocity of 7 ms–1, then what is themomentum of the body?

7. If a force of 50 N acts on a body of mass 10 kg then what is the accelerationproduced in the body?

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248

8. State Newton’s third law of motion. Why it is difficult for a fireman to hold ahose pipe which ejects larger amount of water at a high speed?

9. “Action and reaction forces are equal in magnitude and opposite in direction”.Then, why do they not balance each other?

10. A motorcycle is moving with a velocity of 72 km/h and it takes 6 s to stop afterthe breaks are applied. Calculate the force exerted by the breaks on themotorcycle, if its mass along with the rider is 175 kg.

11. An object of mass 2 kg travelling in a straight line with a velocity of 10ms–1 collides with and sticks to a stationary object of mass 6 kg. Then theyboth move off together in the same straight line. Calculate the total momentumjust before the impact and just after the impact.

12. What is the force of friction? State two methods to reduce friction.

13. What is the relation between thrust and pressure? State the SI units of thrustand pressure. Why a camel can run in a desert easily?

14. A block of wood kept on a table applies a thrust of 49 N on the table top.The dimensions of the wooden block are 40 cm × 20 cm × 10 cm. Calculatethe pressure exerted by the wooden block if it is made to lie on the table topwith its sides of dimensions (a) 20 cm × 10 cm and (b) 40 cm × 20 cm.

ANSWER TO INTEXT QUESTIONS

10.1

1. Yes

2. Pressing a lump of dough with your hands.

10.2

1. When two or more forces acting on an object in opposite direction balances eachother then the forces are known as balanced forces.

2. No. Balanced forces do not change the state of motion of an object.

3. Unbalanced forces acting on an object may change its state of rest or motion.

10.3

1. Due to inertia of rest. When we shake the cloth, the water remains in its positionand comes out.

2. Lower part of our body come to rest but due to inertia of motion our upperpart tends to move in the forward direction and we fall in the forward direction.

Notes


249

Force and Motion


3. As momentum is equal to mass × velocity. So the momentum of loaded truck(more mass) has more momentum.

4. Momentum = m × v = 5 kg × 10 ms–1 = 50 kg ms–1.

5. To decrease the rate of change of momentum boxer moves his head backwardso that the impact of punch is reduced.

10.4

1. Due to force of friction acting between wheel of car and ground.

2. Because the friction between banana peel and ground is very small.

3. Treaded tyres provide better grip with the ground because in such tyres thefriction between the tyres and ground is very large.

10.5

1. Round piece of cloth increases the area of contact between load and head ofporter, thereby decreasing the pressure on his head.

2. To increase the pressure.

3. To decrease the pressure.

4. Nm–2 or pascal (Pa)

Notes


MODULE - 3 GravitationMoving Things

250

11

GRAVITATION

In previous chapter you have learnt that a force is required to change the state ofrest or of motion of a body. You are also aware that all objects when dropped froma height fall towards the earth. Why do objects fall towards the earth? You mightthink that this must be due to some force known as force due to gravity or gravitationalforce. In this lesson we will learn about gravitation, force of gravity and motion ofbodies under the influence of gravity.

We shall also discuss about buoyancy and Archimedes’ principle.

OBJECTIVES


� illustrate the existence of force of gravitation;

� state Newton’s law of gravitation;

� explain the term acceleration due to gravity;

� modify equations of motion of an object falling under gravity;

� solve problems relating to one dimensional motion under gravity;

� distinguish between mass and weight and find the relation between them;

� define free fall motion and explain weightlessness;

� illustrate the force of buoyancy experienced by a body immersed wholly orpartly in a fluid and

� state the principle of Archimedes and apply it to solve problems.

11.1 FORCE OF GRAVITATION

It is our everyday experience that bodies thrown vertically upward come back tothe earth. Even if an object is dropped from some height, it falls towards the earth.Similarly tree leaves and fruits fall toward the earth when they are separated from

Notes


251

Gravitation


the branches. Why does it happen so? This must be due to some force acting onthe bodies like leaves or fruits. What type of force is acting on them? It was IssacNewton who answered this question.

There is an interesting story about Newton. It is said that while Newton was sittingunder an apple tree, an apple fell on him. The fall of the apple set Newton thinking,why did the apple fall down? If some force is acting on the apple then it must bein accelerated motion. Let us try to understand this with the help of an activity.

ACTIVITY 11.1

Release a small stone from your hand from a height of about 1 metre. Observe itsspeed just before it hits the ground. Now, release the same stone from a height ofabout 5 metres (say from first floor of the house) (Fig. 11.1). Again observe its speedjust before it hits the ground. Ensure that in each case the stone is released withoutpushing. Did the stone possess the same speed just before it hits the ground in boththe cases? In which case the stone strike the ground faster? Can you identify theforce which accelerated the stone?

Fig. 11.1 A stone falling from different heights

In above activity you have observed that the force of attraction due to earthaccelerated the stone. Newton knew that bodies fall towards the earth due to forceof gravity. He further thought, if the earth can attract an apple or a stone, can it alsoattract the moon? He was also curious to know whether the same force wasresponsible for keeping the planets go around the sun in their orbits.

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252

Newton concluded that in order to move in a circular orbit the moon must be attractedby the earth continuously. Arguing in the same lines he said that there exists a forcebetween the sun and the planets. The force is known as the gravitational force. Hestated that gravitational force exists everywhere in the universe. All objects in theuniverse attract each other. The interesting aspect of the gravitational force is thatit is always attractive whatever may be the size of bodies.

11.2 NEWTON’S LAW OF GRAVITATION

On the basis of his observations, Newton expressed the law of gravitation in thelanguage of mathematics. He stated the law as follows:

Every particle in the universe attracts every other particle with a force. This forceis proportional to the product of their masses and inversely proportional to the squareof the distance between them. The force is along the line joining the two particles.Mathematically,

Fig. 11.2 Newton’s law of gravitation

F ∝ 1 22

m m

r

where m1 and m2 are the masses of the two particles separated by a distance r.

or F = 1 22

m mG

r...(11.1)

where G is a constant of proportionality. It is called the universal gravitationalconstant. Its value is same everywhere on the earth or in the universe.

In SI units, where m is measured in kilogram, F in newton, r in metre, the acceptedvalue of G is 6.67 × 10–11 Nm2kg–2. As the value of G is very small, you can realizethat the force of gravitation between objects of ordinary mass is very weak.

Let us find out how much the force of attraction between you and your friend sittingon the next bench at a distance of 1 metre apart is. If you are of say 50 kg andyour friend is of 40 kg then the force of attraction would be,

F =116.67 10 40 40

1 1

−× × ××

= 13340 × 10–11 N

= 113.34 × 10–8 N

Notes


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Gravitation


You will appreciate that this force is very weak. It is at least a hundred times weakerthan the force exerted by a small piece of paper on the pan of a balance. You canalso realize how weak the force of gravitation is, when you lift a small stone or whena charged comb picks up small pieces of paper. However, the force of gravitationbecomes appreciably stronger if masses of the objects are increased.

Example 11.1: A boy of 40 kg mass is standing on the surface of earth. If the massof the earth is 6 × 1024 kg and its radius is 6.37 ×106 m, then find the force ofattraction between the boy and the earth. Take the value of G as 6.67 × 10–11

Nm2 kg–2.

Solution: Mass of the earth = 6 × 1024 kg

Mass of the boy = 40 kg

Radius of the earth = 6.37 × 106 m

(This is the distance separating the boy from the centre of the earth)

Value of G = 6.67 × 10–11 Nm2kg–2

The force of attraction (F) between the boy and the earth

=11 24

6 6

6.67 10 6 10 40

6.37 10 6.37 10

−× × × ×× × ×

= 394.5 N

Now you can appreciate that the force with which the earth and the boy attract eachother is more than a thousand million times stronger than the force of attractionbetween you and your friend sitting at a distance of about 1 metre from you.

The gravitational force due to earth is also known as gravity. Thus, when we aredealing with very large masses like the earth, the moon or the sun, the gravitationalforce between such objects is quite large.


1. Why do two students sitting close to each other not feel force of gravitationalattraction between them?

2. Distance between two bodies is increased by a factor of four. How much willbe the change in the force of gravitation?

3. Why is G known as universal gravitational constant?

11.3 ACCELERATION DUE TO GRAVITY

In activity 11.1 we have seen that the speed of a falling stone increases continuously.From this activity we concluded that the stone was accelerated due to force ofattraction between the stone and the earth. Can we give some special name to this

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254

acceleration? This acceleration is called the acceleration due to gravity. Is thisacceleration large if the stone has a large mass? Do heavier objects fall faster thanlighter one? Let us find out.

ACTIVITY 11.2

Caution: while performing this activity, be careful not to hurt anyone.

Ask one of your friends to stand at the roof top of a two storied building with stonesof different masses in his two hands (Fig. 11.3). Ask him to drop these stones together.Carefully observe falling of the stones. What do you find? Why both the stones reachthe ground at the same time?

Fig. 11.3 Dropping two stones of different masses together.

According to a story, Galileo dropped different objects from the leaning towerof Pisa in Italy to prove that objects of different masses fall at the same rate.

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Gravitation


You can perform the above activity in an interesting manner.

ACTIVITY 11.3

Drop a five rupee coin and a paper (15 cm × 15 cm) simultaneously from the sameheight. What do you observe? You will find that the coin falls to the ground muchbefore the paper does. What do you conclude from this observation? You may betempted to conclude that the heavier objects fall faster than the higher ones.

Now crumple the paper into a small ball. Again drop the coin and the crumpled paperball simultaneously from the same height. What do you observe now? You will findthat both the coin and the paper ball hit the ground at the same time. In the firstcase the slowing down of paper was due to friction offered by air. Large surfaceencounters more resistance by air. What conclusion can be drawn from this activity?

This activity shows that two objects of different masses would reach the groundtogether when dropped from the same height. Think why?

A British scientist Robert Boyle placed a coin and feather in a glass tube. Heused a vacuum pump to remove air from the glass tube. When the tube wasinverted both the coin and the feather hit the bottom at the same time.

The earth’s gravity accelerates the coin and paper ball in the downward direction.Since both the coin and paper ball reach the ground together, this acceleration calledacceleration due to gravity (g), is same for both of them. Infact, acceleration dueto gravity is same for any mass at a given place. The SI unit of g is same as thatof acceleration, i.e., ms–2.

Notes



256

Let us try to find out an expression for the acceleration due to gravity. Let the massof the stone falling from a height (in activity 11.1) be m. The acceleration involvedin falling stone due to earth’s gravity is denoted by ‘g’.

We know that force is product of mass and acceleration. Therefore, the magnitudeof force of gravity ‘F’ will be equal to product of mass and acceleration due to gravity.

F = mg (11.2)

From equations (11.1) and (11.2), we have

mg =2

MmG

r

or g = 2

GM

r...(11.3)

where M is the mass of the earth and r is the distance between the object and thecentre of the earth. If the object is on or near the surface of the earth, the distancer in equation (11.3) will be equal to the radius of the earth R. Thus,

g =2

MG

R...(11.4)

Thus we see that the value of ‘g’ is independent of the mass of the freely falling body.The radius of the earth is not same at all the places on the surface of the earth. Sothe value of ‘g’ changes from place to place on the earth. Its value is greater at thepoles than at the equator. The average value of ‘g’ on and near the surface of theearth is taken as 9.8 ms–2.

11.4 MOTION OF AN OBJECT UNDER GRAVITY

We know that g is constant near the surface of earth. Therefore, all the equationsfor uniformly accelerated motion of bodies (discussed in Chapter 9) become validwhen acceleration a is replaced by g. Can you write now the modified equationsof motion? These are:

v = u + gt ...(11.5)

s = 21

2ut gt+ ...(11.6)

v2 = u2 + 2gs ...(11.7)

where u and v are the initial and final velocities and s is the distance covered intime t.

Notes


257

Gravitation


Example 11.2: Take the mass of the earth to be 6 ×1024 kg and its radius as6.4 × 106 m. Calculate the value of g. (G = 6.7 × 10–11 N m2 kg–2).

Solution: From equation 11.4,

g =2

MG

R

=( )

11 2 2 24

26

6.7 10 Nm kg 6 10 kg

6.4 10 m

− −× × ×

×

= 9.8 ms–2

Example 11.3: The mass of the earth is 6 ×1024 kg and that of the moon is7.4 × 1022 kg. If the distance between the earth and the moon is 3.84 × 108 m,calculate the force exerted by the earth on the moon. G = 6.7 × 10–11 N m2 kg–2.

Solution. The mass of the earth, m1 = 6 × 1024 kg

The mass of the moon, m2 = 7.4 ×1022 kg

The distance between the earth and the moon, r = 3.84 × 108 m

G = 6.7 × 10–11 N m2 kg–2

From equation (11.1) the force exerted by the earth on the moon is

F = 1 22

m mG

r

=

( )11 2 2 24 22

28

6.7 10 Nm kg 6 10 kg 7.4 10 kg

3.84 10 m

− −× × × × ×

×

= 2.01 × 1020 N

Example 11.4: A ball is thrown vertically upwards and rises to a height of 122.5 m.Calculate

(i) the velocity with which the ball was thrown upwards and

(ii) the time taken by the ball to reach the highest point.

(Take g = 9.8 ms–2)

Solution: Distance travelled, s = 122.5 m

Final velocity, v = 0 ms–1

Acceleration due to gravity, g = 9.8 ms–2

Notes



258

(i) From equation (11.7) v2 = u2 = 2gs

0 = u2 + 2(–9.8 ms–2) × 122.5 m

For upward motion g is taken as negative.

∴ –u2 = –2 × 9.8 × 122.5 m2s–2

u2 = 2401 m2s–2

u = 49 ms–1

Thus the velocity with which the ball was thrown upwards is 49 ms–1.

(ii) From equation (11.5), v = u + gt

0 = 49 ms–1 + (9.8 ms–2) × t

Therefore, t =49

s9.8

= 5s

Thus,

(i) Initial velocity = 49 ms–1; and

(ii) Time taken = 5s


1. What do you mean by acceleration due to gravity?

2. Why do a heavier and a lighter object when dropped from a same height fallat the same rate?

3. State SI unit of acceleration due to gravity.

4. Write equations of motion of an object moving under gravity.

11.5 MASS AND WEIGHT

11.5.1 Mass

Mass of a body is the quantity of matter contained in the body. Mass of an objectis constant and does not change from place to place. It remains the same whetherthe object is on earth, on moon or anywhere in outer space. The mass of an objectis measured with the help of a pan balance.

We have also learnt in previous chapter that mass of an object is the measure ofits inertia. It means that greater the mass, the greater is the inertia of the object.

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11.5.2 Weight

The weight of an object is the force with which it is attracted towards the earth. Canyou recall the relation between force and acceleration?

Force = Mass × Acceleration

Therefore, F = mg (11.8)

If weight of an object is denoted by W, then

W = mg (11.9)

As weight is a force, therefore, its SI unit is the same as that of the force. Try torecall this unit. It is newton. Its symbol is N. This force (weight) acts verticallydownwards. It has both magnitude and direction. The weight of an object is generallymeasured by a spring balance.

From equation (11.9) we see that weight of an object depends on its mass and valueof g. As the value of g is constant at a given place, therefore, the weight of the objectat a given place is directly proportional to its mass. However, the weight of an objectwill be different on different parts of the earth as the value of g is different on differentparts of the earth.

11.5.3 Weightlessness

You may have noticed increase in weight while in moving in Lift/Elevator upwardand decrease in weight when moving downward. Similar case you can experiencein merry-go-round. Also you have heard that an astronaut experiences weightlessnessin space. What does the term weightlessness mean?

ACTIVITY 11.4

Hold a heavy book in your hand as shown in Fig. 11.4. Can you feel the weightof the book on your hand? Now move your hand quickly in the downward directionwith some acceleration. What do you feel? Do you feel some decrease in the weightof the book? Can you explain the reason for this decrease in the weight?

Book in the hand Hand moving downwardFig. 11.4

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We usually measure the weight by a spring balance or a weighing machine whichrests on a rigid floor. How does a weighing machine record the weight of an object?

Suppose a child is standing on a weighing machine which rests on the floor. The childexerts a downward force equal to his weight W on the machine (Fig. 11.5).

Fig. 11.5 A child on a weighing machine

According to the third law of motion the machine exerts an upward reaction ‘R’ onthe boy which is equal to W. The weighingmachine measures the reaction R, whichis the weight of the boy.

Now imagine that the floor below theweighing machine is suddenly removed.What would happen? The boy and thescale would fall towards the earth withthe same acceleration. In this case theboy cannot exert a force on the weighingmachine. The weighing machine in thiscase would show a zero weight. Thus wecan conclude that a body falling freelyunder gravity is weightless.

Now you can understand why anastronaut experiences weightlessness ina spaceship. The spaceship with theastronaut falls freely towards the earth.The astronaut therefore, appears to befloating weightlessly (Fig. 11.6).

Fig. 11.6 An astronaut in a spaceship

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1. Write two differences between mass of an object and its weight.

2. State two factors on which weight of an object depends.

3. What will be the weight of an apple while it is falling from a tree?

11.6 BUOYANCY AND ARCHIMEDES’ PRINCIPLE

11.6.1 Buoyancy

Have you ever experienced that a mug filled with water appears to be heavier whenit is lifted from bottom of the bucket to above the surface of water than the mugwithin the water in the bucket. Why is it so? Let us understand it with the help ofan activity.

ACTIVITY 11.5

Take a large wooden block and put it in a bucket filled with water. What do youobserve? You will see that the wooden block floats when placed on the surfaceof water.

Now push the block into the water. What do you feel? Why do you feel an upwardpush on your hand? What does it indicate? This indicates that water exerts an upwardforce on the wooden block. Now, push the wooden block further down till it iscompletely immersed in water [Fig. 11.7(a)]. Release the wooden block. What doyou observe? The block bounces back to the surface of water [Fig. 11.7(b)].

(a) (b)

Fig. 11.7 (a) Wooden block immersed in water (b) The block becomesback when released

The upward force exerted by the water on the wooden block is known as the forceof buoyancy or buoyant force. This force is also known as upthrust. In fact, all

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bodies experience a buoyant force when they are immersed in a fluid that is a liquidor a gas. Can you cite some more examples of buoyant force?

What is the magnitude of the buoyant force experienced by an object? Do all objectsin a given fluid experience the same buoyant force? Is not same for all fluids for agiven object? You can answer all such questions after studying Archimedes’ principle.

11.6.2 Archimedes’ Principle

ACTIVITY 11.6

Take a piece of stone and suspend it from a spring balance with the help of a thread[Fig. 11.8(a)]. Note the reading of thespring balance. This is the weight of thestone in air. Now, dip the stone slowly into water kept in a container [Fig. 11.8(b)].Observe carefully. What happens to thereading on the balance?

You will find that the reading of the springbalance decreases as the stone is graduallylowered in water. However, when thestone gets fully immersed in water, nofurther change is observed in the readingof the spring balance. What do you inferfrom this observation? Decrease in thereading of the spring balance shows thatan upward force acts on the stone whenit is dipped in water. As discussed earlierthis upward force is known as the force of buoyancy. Archimedes discovered aprinciple to determine the magnitude of the force of buoyancy.

Archimedes’ principle is stated as follows:

When a body is immersed fully or partially in a fluid, it experiences an upwardforce that is equal to the weight of the fluid displaced by it.

From Archimedes’ principle it is clear that the magnitude of the buoyant force actingon a body at a given place depends on

� density of the fluid and

� volume of the body immersed in the fluid.

Fig. 11.8 Reading of the spring balancedecreases when the stone is

immersed in water

(a) (b)

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Archimedes(287 BC-212 BC)

Archimedes’ principle has many applications. It is used in designing ships andsubmarines. Hydrometers which are used to determine the density of liquids are basedon Archimedes’s principle. Lactometers, which are used for determining the purityof milk, are also based on this principle.

Archimedes was a great Greek mathematician and scientist.He is best known for his famous Archimedes’ principle. Itis said that Archimedes discovered this principle when hestepped in a bathtub full of water and noticed that wateroverflowed from it. He ran through the streets shouting“Eureka”, “Eureka”, ..., which means, “I found it”.

He invented the famous Archimedes’s screw which wasused for raising water from a lower to a higher level. Hiswork in the field of mechanics and geometry made himfamous. About levers once he said that, “give me a bar,long and strong enough, and a place to stand and I willlift the Earth.”


1. Hold a mug full of water inside a bucket filled with water. Now lift it above thesurface of water. Why do you feel it is heavier now?

2. Why does a piece of cork released under water bounce back?

3. What do you mean by buoyant force?

4. Does the buoyant force act on a body when it is kept in vacuum?

5. State two applications of Archimedes’ principle.

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� Newton’s law of gravitation states that every particle in the universe attracts everyother particle with a force, which is proportional to the product of their massesand inversely proportional to the square of the distance between them.

� Force of gravitation between objects of ordinary mass is very weak. However,when large masses are involved this force becomes appreciably stronger.

� The gravitational force due to earth is known as gravity.

� The value of acceleration due to gravity is independent of the mass of the body.

� The weight of an object is the force with which it is attracted towards the earth.It is equal to the product of mass and acceleration due to gravity.

� The mass of an object is constant and does not vary from place to place.However the weight of an object may vary from place to place.

� A body falling freely under gravity is weightless.

� All objects experience a buoyant force when they are immersed in a fluid.

� The magnitude of the buoyant force acting on a body at a given place dependson density of the fluid and volume of the body immersed in the fluid.

� Archimedes’ principle states that when a body is immersed fully or partially ina fluid, it experiences an upward force that is equal to the weight of the fluiddisplaced by it.

TERMINAL EXERCISE

1. State Newton’s law of gravitation.

2. How does the force of gravitation between two objects change when thedistance between them is doubled?

3. How does the gravitational force between two objects change if the masses ofboth objects are doubled?

4. Derive an expression for the acceleration due to gravity on the surface of theearth in terms of earth’s mass, gravitational constant and radius of earth.

5. Write the equations of motion of an object moving or falling only under gravity.

6. What are the differences between the mass of an object and its weight? Onwhat factors does the weight of an object depend?

7. Why does a capped empty plastic bottle released under water bounces backto the surface of water?

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8. What is force of buoyancy? What are the factors on which the magnitude ofthe buoyant force acting on a body at a given place depends?

9. State Archimedes’ principle. Give two applications of Archimedes’ principle.

10. If the average distance between the earth and the sun is 1.5 × 1011 m, calculatethe force of gravitation between the two. Given:

mass of the earth = 6 ×1024 kg

mass of the sun = 2 ×1030 kg

value of G = 6.7 ×10–11 Nm2kg–2

11. What is the mass of an object whose weight is 49N? (Given g = 9.8 ms–2).

12. A stone is dropped from the top of a tower 45 m high. What is its velocitywhen it hits the ground? (Given g = 10 ms–2).

13. A body weighs 3.5 N in air and 2 N in water. How much buoyant force actson the body?

14. A body is immersed in a liquid. If the liquid displaced by the body weighs1 N then what is the buoyant force acting on the body?


11.1

1. Gravitational force is extremely weak. Therefore, small masses do not attracteach other due to this force.

2. As the force of gravitation is inversely proportional to the square of the distancebetween two bodies, the force will decrease by a factor of 1/16.

3. The value of G is same everywhere on the earth or in the universe. Therefore,G is known as universal gravitational constant.

11.2

1. The acceleration produced due to force of attraction by the earth is known asacceleration due to gravity.

2. Because the acceleration due to gravity is same for both heavy and light objects.

3. SI unit for acceleration due to gravity is ms–2.

4. Equations of motion

v = u + gt ...(1)

s =21

2ut gt+ ...(2)

v2 = u2 + 2gs ...(3)

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11.3

1. Mass is the quantity of matter contained in a body.

Man of a body remains the same at all places.

Weight of an object on earth is the force with which it is attracted towards theearth. Weight of an object changes from place to place.

2. Weight of an object depends upon

(i) mass of the body

(ii) acceleration due to gravity.

3. Zero

11.4

1. When immersed in water a buoyant force acts on the mug. Therefore, it feelslighter inside water. When lifted above the surface of water it feels heavier.

2. Due to buoyant force (or upthrust)

3. When an object is immersed in a fluid it experiences an upward force which isknown as buoyant force.

4. No

5. Applications of Archimedes’s principle:

(i) In designing ships and submarines

(ii) Hydrometers or lactometers

MODULE - 4

ENERGY

12. Sources of Energy

13. Work and Energy

14. Thermal Energy

15. Light Energy

16. Electrical Energy

17. Magnetic Effect of Electric Current

18. Sound and Communication

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Sources of Energy


12

SOURCES OF ENERGY

All of us take food for survival and growth of our body. Vehicles like motorcycles,tractors, buses, trucks, ships and aeroplanes require fuel for their running. Even forcooking food we require fuel. Do you know What is important which we get fromthe food or from the fuel? Yes, you are right. It is the energy. From the time youwake up to the time you go to sleep at night, energy plays an important role in yourlife. Energy is important in everyone’s life, whether you notice it or not. Withoutsufficient energy people face difficulties doing their day to day work. All forms ofenergy including solar energy, light energy, mechanical energy, nuclear energy, andthe energy of our body are important to us. The energy of your body enables youto talk, to move and to walk. Is it possible to do any task without energy?

The basic question is: from where do we get all the energy we need? In this lessonwe will learn about different sources of energy, their importance and limitations. Wewill also learn about the energy crisis and how and why it came about? The waysand means of saving and conserving energy in our daily life will also be discussedin this lesson.

OBJECTIVES

After studying this lesson, you will be able to:

� define energy and list various forms of energy;

� identify conventional and non-conventional sources of energy used in India;

� distinguish between renewable and non-renewable sources of energy;

� describe various types of sources of energy e.g. fossil fuels, water, wind,biomass, sea, geothermal, nuclear energy;

� recognise that the sun is the ultimate source of energy;

� explain the advantages and disadvantages of different sources of energy;

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� explain what is energy crisis and how did it develop;

� recognise the need of conservation of energy sources and

� explain the methods of mitigation of energy crisis – energy efficiency andconservation in your daily life.

12.1 ENERGY – AN INTRODUCTION

Energy is a very common word frequently used in our day-to-day life. Energy isdefined as the ability to do work. We require energy for all types of activities includingthe activities within our body, with our body or with other bodies. When we saya body has energy, it means that it is capable of doing work. Look around you willfind countless examples where energy is used to do work. An engine uses energyof its fuel to move a car along. A battery stores the energy needed to switch on theradio or tape recorder. The heavy flow of water can break the banks of rivers asit also has energy in it. Similarly the wind also carries enough energy to shake trees.

12.1.1 Importance of Energy in our Life

Energy plays a very important role in our lives, providing comfort, increasingproductivity and allowing us to live the way we want to. Since the beginning ofmankind, we have made use of wood, water, and fossil fuels as a means of heatingand making machines work. Almost for all types of activities, we rely on one oranother form of energy.

Amount of energy used by a society is an indicator of its economic growth anddevelopment. Without energy even our body would be unable to perform basicfunctions like respiratory, circulatory, or digestive functions to name a few. Plantswould also be unable to complete the process of converting Carbon dioxide, waterand minerals into food without the light from the Sun. Almost all the machines usedfor the production and manufacture of different types of items would be unable tooperate without the use of a source of electrical energy. Almost everything we seearound us, the clothes we wear, the food we eat, the houses we live in, the paperwe write on, the vehicles we drive, all need energy to be created or transformedfrom some natural resource to the final product. Nowadays, the electrical energy hasbecome so important that almost in all walks of life electricity is required. For exampleall electrical appliances in our homes and at our workplace require electricity. Allthe industries and factories run on electricity.

12.1.2 Various forms of Energy

In our daily life we use different forms of energy such as heat energy, light energy,mechanical energy, electrical energy, chemical energy, and sound energy. The most

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common forms of energy are heat, light and electricity. We use all these forms ofenergies for different types of work.

As per requirement, one form of energy can be converted into another form of energyby using specific types of devices or processes. We get energy for our daily use fromdifferent sources. We will learn about details of different forms of energy in otherlessons.

12.1.3 Different Sources of Energy

In simple terms we can say that anything out of which usable energy can be extractedis a source of energy. There is a variety of sources that provide us energy for differentpurposes. You must be familiar with coal, petrol, diesel kerosene and natural gas.Similarly you must have also heard about hydroelectric power, wind mills, solarpanels, biomass etc.

It can be easily seen that some of the energy sources can be replenished in a shortperiod of time. Such energy sources are referred to as “renewable” energy sources,whereas the energy sources that we are using up and cannot be generated in a shortperiod of time are called non-renewable energy sources. Thus, all the sources ofenergy can be divided into two categories: renewable sources and non-renewablesources of energy.


1. List out any five activities from your daily life in which different forms of energyare involved.

2. What are the three most common forms of energy that we use frequently?

3. Differentiate between renewable and non-renewable sources of energy.

12.2 NON-RENEWABLE ENERGY SOURCES

You know that petrol and diesel extracted from crude oil are commonly used to rundifferent kinds of vehicles, such as cars, buses, tractors, trucks, train, aeroplanes etc.Similarly, kerosene and natural gas are used as fuels in lamps and stoves. You shouldalso know that crude oil coal and natural gas occur in limited and exhaustiblequantities. They cannot be regenerated in a short period of time or used again andagain. Hence, they are called non-renewable sources of energy.

It is a fact that at present we get most of our energy from non-renewable energysources which include fossil fuels such as coal, crude oil and natural gas. Lookingat the present and future energy requirements, it is expected that our oil and natural

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gas reserves may last for another 30-35 years (assuming no major new fields arediscovered). Similarly the coal reserves may last no longer than another 100 years.So we must use these non-renewable energy sources judicially and avoid allwastages.

Radioactive elements like natural uranium are also non-renewable. When the atomsof uranium are split into two or more parts, a very large amount of energy is releasedwhich can be used to generate electrical energy.

Let us now, look into details of the fossil fuels as sources of energy.

12.2.1 Fossil Fuels – Conventional Source of Energy

Fossil fuels, such as coal, oil and natural gas, are important non-renewable sourcesof energy. Since the beginning of mankind, we have been using fossil fuels to generateheat, light and electricity for various purposes. These are the primary sources forgenerating electrical energy in the world today. Over 85% of our energy demandsare met by the combustion of fossil fuels. Carbon is the main constituent of thesefossil fuels. Fossil fuels are excellent sources of energy for our transportation needs.You may be surprised to know that approximately 1.9 billion tons of coal is burntin a year to generate electricity in the world. A large amount of chemical energy isstored in the fossil fuels. This stored chemical energy is converted into various otherforms of energy such as heat, light and mechanical energy.

You may be interested in knowing how the fossil fuels are formed? Millions of yearsago the remains of dead plants and animals were buried under the ground. Over theyears by the action of heat from the Earth’s core and pressure from rock and soil,these buried and decomposed organic materials have been converted into fossil fuels.

(a) Coal

Coal is formed in a way similar to the other fossil fuels, though it goes through adifferent process called “coalification”. Coal is made of decomposed plant matterin conditions of high temperature and pressure, though it takes a relatively shorteramount of time to form. Coal is not a uniform substance either; its composition variesfrom deposit to deposit. Factors that cause this deviation are the types of originalplant matter, and the extent to which the plant matter decomposed.

There are different types of coal such as peat, lignite, sub-bituminous and bituminous.The first kind of coal is peat which is merely a mass of dead and decomposing plantmatter. Peat has been used as fuel in the past, as an alternative to wood. Next, thepeat becomes lignite, a brownish rock that contains recognizable plant matter andhas a relatively low calorific value. Lignite is basically the halfway point from peat

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to coal. The next phase is sub-bituminous which is a shade of dull black with verylittle visible plant matter. This type of coal has a less than ideal calorific value.Bituminous coal is the best quality of coal. It. is jet black, very dense and brittle.This type of coal has high calorific value.

Generation of Electrical Energy from Coal

You may be curious to know that how do we get electrical energy from coal?It is basically by means of coal power plants. These power plants first burn thecoal in large furnaces creating tremendous amounts of heat. This heat is used toboil water in boilers so as to convert it into steam. The steam expands, causingpressure to increase in the boiler. A steam turbine is placed at the exit of the boilerso that the moving steam rotates the turbine. In this process the energy from themoving steam gets converted into mechanical energy. The rotating turbine is usedto spin a magnet inside a power generator. This generator is a large electromagnetthat encases the spinning magnet. In this way the electricity is generated and sogenerated electricity is then sent to the national power grid from where it isdistributed in different areas.

(b) Natural Gas

Natural gas is another major source of the energy in our country. Oil and gas fieldshave been found everywhere on the planet except on the continent of Antarctica.These fields always contain some gas, but this natural gas (methane) does not takenearly as long to form. Natural gas is also found in independent deposits within theground as well as from others sources too. Methane is a common gas found inswamps and is also the byproduct of animals’ digestive system.

Although Natural Gas is a fossil fuel, it is cleaner burning than gasoline, but doesproduce Carbon Dioxide, the main greenhouse gas. Like petrol and diesel, naturalgas is also a finite source, though available in larger quantities than the former.

12.2.2 Advantages and Disadvantages of Energy from Fossil Fuels

Use of fossil fuels as sources of energy has both advantages and disadvantages. Letus first take advantages:

� Generation of energy from the fossil fuels technology-wise is easy and relativelycost effective,

� Fossil fuels have a very high calorific value

� Fossil fuels can generate huge amounts of electricity in just a single location.

� Transportation of fossil fuels like oil and gas to the power stations can be madethrough the use of pipe-lines, making it an easy task.

� Power plants that utilize gas are very efficient.

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� Construction of power plants that work on fossil fuels is relatively easytechnology-wise and they can be constructed in almost any location.

If we look into the disadvantages of using fossil fuels, we find that:

� Pollution is a major disadvantage of using fossil fuels as source of energy. Duringthe process of combustion of fossil fuels a lot of toxic gases (and fly-ash in caseof coal) are generated which cause pollution of the atmosphere. These gasesinclude carbon dioxide, which traps the Sun’s heat and may be causing globalwarming. Besides carbon dioxide, coal also gives off sulphur dioxide which maycause acid rain.

� The supply of fossil fuels is limited and cannot be replenished. The rate at whichthey are being consumed, their reservoirs are sure to run out soon.

� Extraction of fossil fuels including coal has resulted in the destruction of wideareas of land and has endangered the environmental balance in some areas

� Mining of fossil fuels including coal is difficult and rated as one of the mostdangerous jobs. Many a times, it endangers the lives of miners

� Use of natural gas can cause unpleasant smell in the area.

The particles formed on burning of fossil fuels are very dangerous. These smallparticles can exist in the air for indefinite periods of time, up to several weeksand can travel for miles. The particles, sometimes smaller than 10 microns indiameter, can reach deep within the lungs. Particles that are smaller than this canenter the blood stream, irritating the lungs and carry with them toxic substancessuch as heavy metals and pollutants. Those affected by these particulates couldbecome afflicted with fatal asthma attacks and other serious pulmonary diseases.

Industrial societies need huge amounts of energy to run their homes, vehicles andfactories. More than 80% of this energy comes from burning coal, oil and naturalgas. These are called fossil fuels, because they formed from the remains of plantsand tiny sea creatures that lived on Earth many millions of years ago. They includefuels made from oil, such as petrol, diesel and fuel for jet planes.

12.2.3 Energy from the Atom – Nuclear Energy

The atoms of a few elements such as radium and uranium act as natural source ofenergy. In fact atoms of these elements spontaneously undergo changes in which thenucleus of the atom disintegrates.

Let us see how we get energy from the atom. You should know that a large amountof energy is stored in the nucleus of every atom. The energy stored in the nuclei ofatoms can be released by breaking a heavy nucleus such as uranium into two lighter

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nuclei. The splitting of the nucleus of anatom into fragments that are roughly equalin mass with the release of energy is callednuclear fission. (A small amount of eachfission mass vanishes, in releasing hugeamounts of energy as per E = mc2, wherem is the missing mass and c is the velocityof light). When a free neutron strikes aUranium (235) nucleus at a correct speed,it gets absorbed. A Uranium (235) nucleuson absorbing a neutron becomes highlyunstable and splits into nuclei of smalleratoms releasing huge amount of energy inthe process. During this process, a fewneutrons are also released. These neutronssplit other nuclei of the Uranium (235).The reaction continues rapidly and is knownas the chain reaction. In this process alarge amount of energy is released. Thisenergy is used for boiling water till it becomes steam. Steam so generated is usedto drive a turbine which helps in generating electrical energy.

The fission reaction is carried out in a controlled and regulated manner in nuclearreactors. (Else, they would explode like bombs with an uncontrolled chain reaction.)In order to control the fission reaction, some of the neutrons released by the reactionare absorbed by the control rods made of boron / cadmium. In our country nuclearreactors are functioning at Tarapur, Kalpakkam, Kota and Narora for generatingelectricity.

If the nuclear chain reaction is uncontrolled, all the nuclei in the piece of uraniumsplit in a fraction of a second and this may cause a devastating explosion – suchas those of the atom bombs dropped on Hiroshima and Nagasaki in Japan byAmerica.

(a) Uses of Nuclear Energy

Nuclear energy is non-renewable as the uranium fuels used are consumed in the fissionreaction and hence are non replenishable. Nevertheless, nuclear energy has manyuses:

(i) Energy produced in a nuclear reactor can be harnessed to produce electricity.

(ii) Nuclear energy is also being used to power submarines and ship. Vessels drivenby nuclear energy can sail for long periods without having to refuel.

Nuclear Fusion

Energy is also produced when twolight nuclei such as deuterium (heavyhydrogen) combine together to forma heavy nucleus. A process in whichthe nuclei of light atoms are combinedto form a nucleus of a heavier atomwith the release of energy is callednuclear fusion.

Nuclear fusion requires very hightemperature, say of the order of 4million degree Celsius (4000000 °C).This is the mechanism through whichenergy is produced in stars, includingour sun. This reaction has been usedto make hydrogen bombs.

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(iii) Radioisotopes obtained as by-products in nuclear reactions are used inmedicine, agriculture and research.

(b) Hazards of Nuclear Energy

On one side nuclear energy seems to be an alternative to fossil fuels, on the other,it can also be hazardous. Nuclear radiations and the radioactive wastes are two majorhazards that accompany production of nuclear energy. Let us know little more aboutthem.

1. In the process of producing nuclear energy, harmful nuclear radiations may getaccidentally leaked/released which can penetrate human bodies and causeirreparable damage to cells. For preventing this from happening, nuclear reactorsare covered with a thick shell of radiation absorbent material such as lead.However, accidental releases of these extremely harmful radiations into theenvironment pose a constant threat to those inhabiting the surrounding areas.Perhaps you may be aware of the two major accidents in nuclear power plants– one at the Three Mile Island (U.S.A.) and the other at Chernobyl (the thenSoviet Union). The immediate devastation caused in these two accidents throughthe release of harmful nuclear radiations was huge and its full extent is yet to beassessed.

2. Another hazard relate to the problems involving disposal of harmful radiant wastesmainly spent fuels produced in the fission process. During nuclear reactions, anumber of harmful substances capable of emitting nuclear radiations aregenerated. These substances are called nuclear wastes. Presently, most of thenuclear waste generated in nuclear power plants is simply being storedunderground in strong lead containers. We have not yet been able to discoversafer and more satisfactory methods of disposing the nuclear wastes.

There are major advantages of using nuclear energy over fossil fuels.

� Unlike fossil fuels, the nuclear fuel used in nuclear power stations, do not burn.Hence no waste gases are produced.

� Small amounts fuel materials, yield huge amount of energy.


1. Name any four non-renewable sources of energy and give at least one advantageof each.

2. Nuclear energy is considered to be a very powerful alternative of fossil fuels.Even then why is it not being used on a much larger scale?

3. What are the limitations of using natural gas for meeting our energy requirements?

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12.3 RENEWABLE ENERGY SOURCES

You have learnt in the previous section that the fossil fuels such as coal, oil and naturalgas meet most of the energy needs of the world today. But what will happen whenthe reserves of these non-renewable sources of energy get completely exhausted?,We also need to pay attention to the damaging effects of fossil fuels on theenvironment.

The solution, surely, must lie in switching to alternative sources of energy andenvironment-friendly natural fuels. There are several alternative and renewablesources of energy which are not only environment friendly but can also be availablein abundance. Water, wind, sunlight, geothermal, sea waves, hydrogen and biomassare some such possible sources of energy. In addition to the renewability, there areother reasons why we should look to switching over to such sources. Such as:

� To reduce pollutants, greenhouse gases and toxins that are by-products of non-renewable sources of energy;

� The use of alternative energy sources can help preserve the delicate ecologicalbalance of the earth, and help conserve the non-renewable energy sources likefossil fuels; and

� Renewable sources are inexhaustible.

Fortunately there are many means of harnessing renewable sources of energy whichhave less damaging effects on our environment. Here are some possible alternativesin the next sub sections.

12.3.1 Sun - The Ultimate Source of Energy

The sun has been providing us heat and light for billions of years and it is expectedthat it will continue to do so for billions of years to come. All plants get their energyfrom the sun and all animals get their energymainly from the plants. Therefore, it may beconcluded that sun is a source of energy foranimals. Even the energy stored in butter, milkand eggs comes form the sun. Why do wesay so? The sun in fact is the ultimate sourceof energy for all living beings. Apart fromnuclear energy, all other forms of energyresult from solar energy. It is said that thefossil fuels, bio-fuels and natural gas are ineffect “bottled” solar energy. The wind andrivers which provide renewable energyare also the result of solar energy. Can youthink how?

Fig. 12.1

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Sun is one of the most powerful renewable sources of energy for the future. As longas the sun exists, we will continue to get its energy. About 30% of the incoming solarradiation is absorbed by the upper atmospheres, the rest is absorbed by the land,sea and clouds.

Fig. 12.2 Photovoltaic Water Pumping

Solar energy is used commonly for heating, cooking, production of electricity, andeven in the desalination of seawater. With the help of solar cells, solar energy isconverted into electricity. One of the most common uses of the sun’s energy has beenfor water heating systems. It is also used to provide power to the vehicles, generateelectricity, lighting streets, cooking etc. On a small scale, solar energy is being usedto heat up water for daily use in our homes and also the swimming pools. On a largerscale, solar energy could be used to run cars, power plants, and space ships etc.

Fig. 12.3 Solar flat plate collector Fig. 12.4 Box type solar cookers

(a) Advantages of Using Solar Energy

We have been using the light and heat of the sun’s rays since ancient times for differentpurposes. Some of the advantages of using solar energy are:

� Use of solar energy causes no environmental pollution, because no chemicalwaste or toxic gases get released while using solar energy,

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� Solar energy can be used for practical purposes such as heating and lighting,

� The sun is an ever lasting source of energy which is freely available, and

� Can be converted into electrical energy and put to many uses.

(b) Limitations of Using Solar Energy

No doubt, the sun is the source of all the energies in one way or another, but usingthe sun as a source of energy also has certain limitations. Firstly, solar power plantscan not produce energy if the sun is not shining. For example during night time andcloudy days it is not possible to produce energy from the sun. Secondly, establishmentof solar power stations can be very expensive. Thirdly the solar panels need to beregularly maintained and cleaned to continue generating electricity.

12.3.2 Wind Energy

Wind power is another alternative energy source thatcould be used without producing by-products that areharmful to nature. Like solar power, harnessing the windis highly dependent on weather and location. However,it is one of the oldest and cleanest forms of energy andthe most developed of the renewable energy sources.There is the potential for a large amount of energy tobe produced from windmill.

You must have seen a phirki. It is also called a wind-vane. What happens when you blow air on the bladesof phirki? It starts rotating. Using phirki, you can easilyexperience that wind provides energy.

(a) Advantages of Wind Energy

� Wind energy is free of cost and reliable,

� Wind power is clean and produces no environmentalpollution,

� In wind power generation no harmful by-productsare left over as in case of burning of fossil fuels,

� Since wind is a renewable source of energy, we never run out of it,

� Farming and grazing can still take place on land occupied by wind turbines whichcan help in the production of bio fuels. When used inland, the land beneath thewindmill can still be used for farming purposes.

� Wind farms can be built off-shore.

� In some cases wind farms can even be tourist attractions.

Fig. 12.6 Phirki

Fig. 12.5 Windmill

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(b) Limitations of Wind Energy

� Wind power is not available all the time, at all the places and has to be usedwhile being produced, as it cannot be stored.

� Persistent wind and consistent wind speeds are needed for continuous powergeneration. If wind speed decreases, the turbine lingers and less electricity isgenerated.

� The wind farms, whether onshore or off shores are unsightly, noisy and generatea lot of opposition.

� Large wind farms can have a negative effect on the scenery.

� They are hazards for wildlife, especially birds who commonly fly into their blades.

Different parts of our country, which are windy most of the time, have windmills topump water and generate electricity. These wind mills are big wind-wanes in whichwind energy is used. Let us look into the working of a windmill.

(c) Working of Windmill

A windmill is basically a mechanical arrangement to convert wind energy into anotherform of energy. It has blades. The blades of the windmill rotate in a vertical planewhich is kept perpendicular to the wind. As wind flow crosses the blades of thewindmill, the blades start rotating. The rotation of blades makes the turbine rotate.The turbine is attached with an electrical generator which converts mechanical energyof the turbine into electrical energy. The blades are angled into the wind, so as torotate in a way which maximize the generation of electricity.

In older windmills, wind energy was used to run machinery to do physical work,like crushing grain or pumping water. Wind towers are usually built together on windfarms. Now, electrical currents are harnessed via large scale wind farms that are usedby national electrical grids, as well as small individual turbines used for providingelectricity to isolated locations or individual homes.

The wind speed is vital in the production of electricity, and the optimum speed isapproximately 25 km/h and this causes the blades to rotate.

12.3.3 Hydroelectric Energy

Like wind energy, the flowing water and water stored in huge dams is also a veryimportant source of energy which is known as hydroelectric energy. But, over-development and unrestricted harnessing of water power can have a devastatingeffect on the local environment and habitation areas.

(a) Generation of Hydroelectricity

Hydroelectric is produced by the natural flow or fall of water. By channelling waterthat is flowing downhill, the force of the water can be used to turn turbines and viaa generator, produce electricity.

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Hydroelectricity comes from the damming of rivers and utilizing the potential energystored in water. As shown in the Fig. 12.7, when the water stored behind a damis released its potential/kinetic energy is transferred onto turbine blades and used togenerate electricity. Though the initial cost of setting up of hydroelectric power systemis high, it has relatively low maintenance costs and provides relatively inexpensivepower.

Fig. 12.7 Generation of Hydroelectricity

The power output of the hydroelectric source is determined by the difference in heightbetween the source and the outflow. This height difference is known as the head andthe greater the head, the larger the output. For this purpose, very big dams are madeon the rivers and other water flows.

(b) Advantages of Hydroelectric Power

� It is a source of renewable energy in the form of hydroelectric power.

� It is cost effective and is competitively productive against non renewable sources.

� Electricity can be generated constantly, because there are no external factors,which affect the availability of water.

� Hydroelectric power produces no waste or pollution since no chemicals areinvolved.

� Water used for hydro power can be reused for other purposes/like irrigation etc.

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(c) Limitations in Using Hydroelectric Power

Though water is an excellent source of generating electricity, it also has certainlimitations:

� The hydroelectric power plants cannot be sited at a place of our choice. Theremust be a strong current or considerable height to make the productionworthwhile, as the capital cost of setting up production is relatively quite high.

� Dams can be very expensive to build.

� There needs to be a sufficient, and continuously strong water current, or waterhead, to produce energy.

12.3.4 Geothermal Energy

Geothermal energy is another alternative source of energy. Geothermal energy isobtained from the internal heat of the earth. In fact it is one of the oldest types ofnatural sources of heat. It dates back to Roman times, when the heat from the earthwas used instead of fire, to heat rooms and/or warm water for baths. Presently itis being used as a source for producing electricity, mainly in regions of tectonic platemovement.

Now the basic question is how do we get geothermal energy? You must have heardabout the volcanoes found around the world. These volcanic features are calledgeothermal hotspots. Basically a hotspot is an area of reduced thickness in the mantlewhich expects excess internal heat from the interior of the earth to the outer crust.These hotspots are well known for their unique effects seen on the earth’s surface,such as the volcanic islands, the mineral deposits and geysers (or hot springs). Theheat from these geothermal hotspots is altered in the form of steam which is usedto run a steam turbine that can generate electricity.

(a) Advantages of Geothermal Energy

Geothermal energy is used for heating homes and for generating electricity withoutproducing any harmful emissions. Some of the advantages of using geothermal energyare:

� Unlike most power stations, a geothermal power plant does not create anypollution. Harnessed correctly, it leads to no harmful by-products.

� Geothermal Power plants have very low running costs. Because they requireenergy to run a water pump (which is provided by the power plant itself).Moreover, there are no costs for purchasing, transporting, or cleaning up of fuels.

� Geothermal power plants are an excellent source of clean, and inexpensiverenewable energy.

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� Geothermal energy can be used to produce electricity 24 hours a day.

� Geothermal power plants are generally small and have little effect on the naturallandscape, or the near environment.

(b) Limitations of Using Geothermal Energy

Though geothermal energy has several advantages, it also has limitations:

� If harnessed incorrectly, geothermal energy can produce pollutants.

� Improper drilling into the earth can release hazardous minerals and gases.

� Geothermal power plant sites are prone to running out of steam in the long run.

The Earth can be divided into three large sections: the mantle, the inner core, andthe outer core. The inner core is at the center of the earth. The pressure andtemperature increase as one move closer to the center of the earth. As one movesoutwards from the inner core, one encounters the outer core and then mantlefollowed by the crust. The mantle is a layer that is below the crust of the earth.This is said to go down 2,900 kms; its temperature is about 870 degrees Celsius.The outer core has a very high temperature which ranges from about 4,400degrees Celsius to about 6,100 degrees Celsius. The outer core begins wherethe mantle ends and it extends further down to the center 2,250 kms. The innercore is about 6,400 km below the earth’s surface. The temperature at the innercore of the earth is at the high of about 7,000 degrees Celsius. The hightemperature of the earth’s core is the basic reason behind geothermal energy.

12.3.5 Ocean – A Source of Energy

You may be surprised to know that the ocean is also a powerful source of renewableenergy. The energy of the ocean can be harnessed in three basic ways: using wavepower, using tidal power, and using ocean water temperature variations. Let us studyeach of these, one by one.

(a) Using Ocean Wave Power to Generate Energy

You may know that different types of waves are continuously generated in the ocean.The back-and-forth or up-and-down movement of waves can be captured to harnessthe wave power by using it to force air in and out of a chamber to drive a pistonor spin a turbine that can power a generator. In fact, kinetic energy exists in the movingwaves of the ocean. That energy can be used to power a turbine as shown in Fig.12.8.In this figure you can see that when the wave rises into a chamber, it forces the air

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out of the chamber. The moving air spins a turbine which can turn a generator. Whenthe wave goes down, air flows through the turbine and back into the chamber throughdoors that are normally closed.

Fig. 12.8 Generation of Ocean Energy

This is only one type of wave-energy system. Others actually use the up and downmotion of the wave to power a piston that moves up and down inside a cylinder.That piston can also turn a generator. Presently in some cases the wave power isbeing used in small lighthouses and warning buoys.

(b) Using Tidal Power of Ocean to Generate Energy

The tidal energy of ocean can also be harnessed by trapping water at high tide andthen capturing its energy as it rushes out and drops to low tide. When tides comeinto the shore, they can be trapped in reservoirs behind dams. And when the tidedrops, the water behind the dam can be let out just like in a regular hydroelectricpower plant. Presently, the power of the tides is being harnessed to produce electricityin Canada and France.

(c) Using Ocean Water Temperature Variations to Generate Energy

If you go swimming in the ocean and dive deep below the surface, you will noticethat the water gets colder the deeper you go. It is warmer on the surface becausesunlight warms the water. But below the surface, the ocean gets very cold. That iswhy scuba divers wear wet suits when they dive down deep. Their wet suits traptheir body heat to keep them warm.

This temperature difference between deep and surface waters in the ocean is alsoused to extract energy from the flow of heat between the two. The process is called“ocean thermal energy conversion” (OTEC). Power plants can be built that use thisdifference in temperature to generate energy. Presently, it is being used in Japan andin Hawaii in demonstration projects.

Air Back in Air out

TurbineGenerator

Wave Direction

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(d) Advantages and Disadvantages of Using Ocean Energy

The energy potential of an ocean, particularly tidal basins, is large. The ocean energyis preferable to that of wind because tides are constant and predictable and thatwater’s natural density requires fewer turbines than are needed to produce the sameamount of wind power. However, tidal energy systems can have environmentalimpacts on tidal basins because of reduced tidal flow and silt build up.

12.3.6 Energy from Biomass

You may know that the biomass is organic material made from plants and animals.It includes garbage, industrial waste, crop residue, manure, wood, sewage and deadparts of living objects. Like all other sources of energy, it also contains stored energyfrom the sun. Therefore, biomass is also a very good source of energy.

Do you know how biomass contains sun’s energy? You know that the plants absorbsun’s energy in a process called photosynthesis. The chemical energy in plants getspassed on to animals and people who eat them. On burning the biomass, the chemicalenergy stored in it is converted into heat energy. The thermal energy released frombiomass can be used to provide heat to industries and homes, and also to producesteam for generating electricity. But you have learnt by now that on burning any typeof fuel, harmful emissions are released. So how biomass can be a good source ofenergy? Can we get energy from biomass without burning?

Yes. Burning biomass is not the only way to release its energy. Biomass can beconverted to other useable forms of energy, such as biogas or methane, ethanol andbiodiesel. You have learnt earlier that methane is the main ingredient of natural gasas well. The smelly stuff like rotting garbage, and agricultural and human wastesrelease methane gas - also called “landfill gas” or “biogas”. Like liquid petroleumgas (LPG), the biogas is also used for cooking and lighting.

Biofuel including biogas and bio-diesel is another important fuel produced from left-over food products like vegetable oils and animal fats. Biofuel is made mainly in twoways. The first is when large amounts of crops high in sugar or starch content aregrown, and then fermented with yeast to produce ethyl alcohol or ethanol. Plantslike corn, soybeans, rapeseed, wheat, sugar cane and sugar beet are used to produceethanol. Ethanol can be used as an alternative fuel in petrol engines, but it is verycorrosive and so can be harmful to engine parts and components. The other optionis that it can be mixed with petrol to produce a more bio-friendly fuel which canbe used in engines. In the second method, plants high in vegetable oils are grownand then the vegetable oil is processed to produce bio-diesel.

Thus we can say that biomass can be used as a source of energy in the followingthree ways:

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� by burning dry biomass directly to produce heat, or generate steam.

� by decomposition of biomass in the absence of oxygen to produce methane gas.

� by producing bio-diesel from the plants high in vegetable oils.

(a) Advantages of Using Biomass as Source of Energy

Biomass is an inexhaustible energy source because we can always grow more treesand crops, and waste will always exist. Using biomass as a source of energy hasfollowing advantages:

� When direct combustion of biomass is not used to generate energy, there is hardlyany environmental impact.

� Biodiesel and other fuels produced by biomass are viable and a clean sourceof energy.

� Biomass is easily available throughout the world.

� The residue from biomass plants can be used as manure.

(b) Limitations of Using Biomass as Source of Energy

Though biomass is a clean and renewable source of energy, it has certain limitations.Some of them are:

� The bio-fuel or ethanol produced from biomass is not as energy efficient as petrol.

� If the biomass is directly burnt, it may contribute to global warming and increaseemissions causing environmental pollution.

� The main ingredient of biofuel i.e. methane is harmful to the environment.

� Biomass is a relatively expensive source for generating energy, both in terms ofproducing the biomass and converting it to ethanol.

12.3.7 Hydrogen – A Future Source of Energy

Hydrogen could be a very environmentally friendly source of energy in the future.In the long-term, hydrogen is likely to reduce dependence on conventional sourcesof energy such as petrol, diesel and coal etc. In addition to it, the use of hydrogenas source of energy will help in reducing the emission of greenhouse gases and otherpollutants.

When hydrogen is burned, the only emission it makes is water vapour, so a keyadvantage of hydrogen is that when burned, carbon dioxide (CO2) is not produced.Thus, we can say that hydrogen does not pollute the air. Hydrogen has the potentialto run a fuel-cell engine with greater efficiency over an internal combustion engine.The same amount of hydrogen will take a fuel-cell car at least twice as far as a carrunning on gasoline.

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Though, the hydrogen fuel cell has proved to be a viable source of energy for vehicles,but there are serious questions on its production, storage and distribution. There arealso questions on its efficiency, in so far as it takes more energy to manufacture itthan what it produces. Besides, it costs a considerable amount of money to run ahydrogen vehicle because it takes a large amount of energy to liquefy the fuel.

Hydrogen is one of the most abundant elements in the universe. It is the lightestelement, and it is a gas at normal temperature and pressure. Hydrogen as a gasis not found naturally on Earth, because hydrogen gas is lighter than air and risesinto the atmosphere. Natural hydrogen is always associated with other elementsin compound form such as water, coal and petroleum.


1. Name any one alternative source of energy which you would like to use in yourhome. Justify your answer.

2. Biofuel is considered to be a good fuel. Why is it not being used on a mass scaleto replace the fossil fuels in our country?

3. List any five traditional uses of solar energy.

12.4 TRANSFORMATION OF ENERGY

As you have learnt earlier, energy can exist in many different forms. It is also truethat energy can be changed from one form to another. But it cannot be created ordestroyed. Normally, we talk about ‘using energy’, but do you know, it never gets‘used up’. It just gets transformed into another form. Eventually, most of it ends upas heat, but it is so spread out that it cannot be detected or used.

Let us see how transformation of energy takes place in our day-to-day life. Someexamples are:

� The food has chemical energy stored in it. When our body uses this stored energyto do some work, it gets converted in to kinetic energy. Similarly, when you kicka ball, your muscles change chemical energy from your food into kinetic energy.As the ball moves through the air and across the ground, friction slows it downand its kinetic energy is changed into thermal energy (heat).

� A car uses stored chemical energy in petrol or diesel to move. The engine changesthe chemical energy into heat and kinetic energy to power the car. Things thatare moving, such as vehicles, flowing water, and winds etc. have kinetic energy.

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� In a thermal power station, the chemical energy of coal is transformed into heatenergy of the hot steam, and then into mechanical energy of turbine. Thismechanical energy is transformed by a generator into electrical energy, whichpasses through the power lines to various places – cities, towns, houses, factoriesetc., where it is transformed back to heat, light, sound or mechanical energy.

� Spring or other stretched or compressed materials have potential energy.

� The water stored in dams and reservoirs also has potential energy which getsconverted in other forms of energy.

� When hot materials cool down, they give off heat, or thermal energy. The fuelsand batteries have chemical energy stored in them. When they are used theirenergy is released by chemical reactions.

� When you talk on the phone, your voice i.e. sound energy is transformed intoelectrical energy, which is transmitted through wire or the air. The phone on theother end changes the electrical energy into sound energy through the speaker.Similarly, a television changes electrical energy into light and sound energy.

As per the “Law of Conservation of Energy”, energy can neither be created nordestroyed, it can only be transformed from one form of energy into another. Detailsabout the transformation of energy will be discussed in another lesson.

12.5 ENERGY CRISIS AND ITS MITIGATION

All activities, small or big, need one or another form of energy. We can say that theenergy is the lifeline for our survival and development. Because of paucity of electricalenergy, some Indian households, particularly in rural areas, go without electricity fordays. Even in urban areas the situation is not very good. There are frequent electricitycuts for several hours during a day. This becomes a severe problem during thesummer. Energy demand in the future will continue to increase as India’s populationand its needs continues to grow.

The situation in which a country suffers from frequent disruptions in energy suppliesbecause of large and increasing gaps between availability and demand of electricityaccompanied by rapidly increasing energy prices that threaten economic and socialdevelopment of the nation may be termed as the energy crisis. Energy crisis is beingfaced by all developing nations including India. What are the reasons behind suchas energy crisis?

12.5.1 Reasons behind Energy Crisis

It is a fact that presently around 85 percent of the world’s energy supply is met fromoil, coal and natural gas. Clearly, we live in the age of coal and oil, but the availability,of both of these resources of energy are very limited and will not last beyond a fewdecades. If we think about the Indian situation only, coal accounts for over 70%

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of India’s energy production. However, it is a limited resource and also createsenvironmental problems. Even if more coal is mined, the increasing gap between thedemand and supply of energy in India can not be easily bridged. Indian villagers areforced to spend from two to six hours per day gathering fuel for their householdcooking needs. Moreover, India’s reliance on firewood has led to deforestation andpollution. Thus, the basic reasons behind over energy crisis seem to be the following:

� Our over-dependence on limited and exhaustible sources of energy such as ourcoal and oil deposits.

� Increasing gap in the demand and supply of the energy.

� Ever increasing prices of the energy and fuel from other countries.

� Reluctance in using alternative and renewable sources of energy, such as solar,wind, bio-energy, etc..

� Overuse and misuse of the available sources of energy.

12.5.2 Methods of Mitigating Energy Crisis

In order to mitigate the problem of energy crisis, the Government as well as the peopleof the country should take collective and serious steps.

(a) It is believed that one possible solution to India’s energy problems is NuclearPower. Accordingly, we signed a Nuclear Deal to import fuel and technology.The model of nuclear powered energy has been successful in countries likeFrance where they meet more than 75% of their electricity requirements fromnuclear energy.

(b) The use of renewable sources of energy like solar power, wind power,hydroelectric power, biogas and biofuel etc should be promoted. As automobilesare major consumers of petroleum fuels/oils, an effort should be made toincrease the mileage standards of the automobiles. Even the generation of energyfrom renewable sources is not very simple and cost effective. Therefore, all ofus should make a sincere effort to save and conserve the energy.

(c) Being an agricultural nation we could have come up with a more ingenioussolution to produce ethanol and biofuel from sugarcane and vegetable oils.

In addition to the above initiatives to solve the problem of energy crisis, we shouldfollow an ‘energy conservation approach’ in our daily life. Some useful tips, on howwe can save energy in our daily life, are given in the following section.

12.5.3 Conservation of Energy

The key for resolving the country’s energy crisis lies with us citizens. Among thingswe can do is the conservation of our non-renewable sources of energy. It is said

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that energy saved is as good as energy generated. Therefore, we should not onlyjudiciously use energy sources but save energy as much as we can. You can startconservation of energy in your home. Some of the important tips for saving energyare:

� Switch off lights, fans and other appliances when not in use. Water taps shouldnot be left open.

� While cooking rice, dal etc. the vessel should remain covered and, for cooking,only the required quantity of water should be used. If you soak pulses in waterfor some time before cooking, it will save energy in cooking.

� Another way of saving energy is by use of more efficient appliances. For example,a LED or CF light is much more efficient that a tube light or bulb; and a tube-light gives much more light than a bulb of same power rating. In fact, bulbs arebeing totally phased out in some countries. Better stoves burn fuel efficiently andgive more heat per unit of fuel burned. The fuel efficient vehicles should be usedand their engines should be maintained properly. Similarly, more energy efficientelectrical appliances having energy saving stars should be used,

These are only some of the habits which can save a lot of energy. We should findways for not wasting energy where it can be saved. For example, if you are requiredto go to a nearby place you may walk or go by a bicycle and avoid the use of anautomobile. You may use public transport in place of your own vehicle to save fuel.Share automobiles rides to office, instead of driving alone to office.


1. What are the steps that you can and should take for saving energy at home orin the office?

2. List at least three reasons behind the energy crisis in our country.

3. What do you mean by the statement that ‘energy can neither be created nordestroyed?


� All processes taking place on the earth require energy. Energy is the ability todo work.

� The sun is considered to be ultimate source of energy for life on earth. We alldirectly or indirectly use sun’s energy which is also called solar energy.

� Coal and petroleum are fossil fuels. Presently, they are the main source of energyin our country.

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� Energy sources are either renewable or non-renewable. Non-renewable sourcesare getting depleted.

� We should try to utilize renewable sources of energy in order to conserve fossilfuels and also to protect our environment.

� Energy exists in various forms. Energy can be transformed from one form toanother. Energy can neither be created nor destroyed. In any energy transformation,the sum total of energy remains contant.

� Fission is a process of splitting up of the nucleus of a heavy atom into fragmentsof roughly of equal masses. Huge amount of energy is released in the processof nuclear fission, where the missing mass gets converted into energy (videE = mc2).

� In order to conserve energy we should not only judiciously use energy sources,we should also save energy as much and as far as we can.

TERMINAL EXERCISE

1. What are different forms of energy?

2. Distinguish between conventional and non-conventional sources of energy.

3. What are conventional sources of energy? Give two examples.

4. Why non-conventional sources of energy are preferred over the conventionalsources?

5. “Sun is the ultimate source of energy”. Justify this statement.

6. List some uses of nuclear energy.

7. What are the hazards of producing nuclear energy?

8. What do you mean by the energy crisis? List out the possible reasons.

9. What measures should be taken to mitigate the problem of energy crisis in ourcountry?

10. Why should we save energy?


12.1

1. (i) Cooking of food – heat energy and chemical energy of fuel

(ii) Lightning of bulbs – electrical energy and light energy

(iii) Talking to each other – sound energy

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(iv) Cycling – mechanical energy

(v) Torch – chemical energy of cells

2. (i) Heat, (ii) Light and (iii) Electricity

3. The energy sources that can be replenished in a short period of time are calledrenewable energy sources, whereas the energy sources that we are using up andcannot be generated in a short period of time are called non-renewable energysources.

12.2

1. (i) Coal, Advantage: It is cheaper and easily accessible.

(ii) Oil, Advantage: It is excellent sources of energy for our transportation.

(iii) Natural Gas, Advantage: It is cleaner burning than gasoline, but doesproduce Carbon Dioxide, the main greenhouse gas and it has high calorificvalue.

(iv) Nuclear Fuel, Advantage: Nuclear fuel used in nuclear power stations doesnot burn and hence no waste gases are produced.

2. Because of the following reasons:

It is difficult to set up nuclear power plants and also a lot of money has to bespent on safety of the nuclear power plants. Moreover the nuclear wasteproduced from plants can be hazardous.

3. Limitations of using natural gas for meeting our energy requirements:

Stock of natural gas is limited and it cannot be replenished.

Use of natural gas can cause unpleasant smell in the area.

12.3

1. Solar energy. Because it is free and easily available in the area in which we live.It can be used for cooking, water heating and also for keeping our home warmin winter.

2. (i) The bio-fuel is not as energy efficient as petrol.

(ii) The main ingredient of bio-fuel i.e. methane is harmful to the environment.

(iii) Bio-fuel is a relatively expensive source for generating energy, both in termsof producing the biomass and converting it to ethanol.

3. Traditional uses of solar energy.

(i) drying of clothes (ii) heating of water (iii) drying crops,

(iv) breeding and raising chicks and (v) drying manure

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12.4

1. Steps for saving energy

� Switch off lights, fans and other appliances when not in use.

� Water taps should not be left open.

� While cooking vegetables the vessel should remain covered.

� For cooking, only the required quantity of water should be used.

� Soak pulses in water for some time before cooking,

� use of more efficient appliances.

� use public transport in place of your own vehicle to save fuel.

� Share automobiles rides to office, instead of driving alone to office.

2. Reasons behind the energy crisis in our country.

� Our over-dependence on limited and exhaustible sources of energy such asour coal and oil deposits.

� Increasing gap in the demand and supply of the energy.

� Ever increasing prices of the energy and fuel from other countries.

� Reluctance in using alternative and renewable sources of energy, such as solar,wind, bio-energy, etc..

� Overuse and misuse of the available sources of energy.

3. The statement ‘energy can neither be created nor be destroyed’ means thatenergy the total energy remains constant. It can only be transformed from oneform of energy into another.

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13

WORK AND ENERGY

In previous lesson we have learnt that force changes the motion (i.e. momentum).But during the change in motion the body on which the force is applied also movesthrough some distance. This leads us to some more basic concepts of science - work,power and energy, which we will discuss in this lesson.

We commonly use the word like work and energy in our daily life. Let us study thelesson and found out how science define these terms.

We will also come to know in this chapter about the various forms of energy, examplesof their interconversion and the most basic law of nature which governs these energytransformations – the law of conservation of energy.

Sometimes we want the work to be done more quickly. The quantity which measuresthe rate at which work is done is called power. Performance of a machine is usuallyrated by power.

OBJECTIVES

After studying this lesson you will be able to:

� define the terms work and energy and their SI units;

� compute work done by a constant force;

� list various forms of energy-like mechanical, thermal, light, sound, electrical,chemical, and nuclear energy with examples;

� define and explain potential and kinetic energy with suitable examples;

� cite examples of transformation of energy;

� state and explain the law of Conservation of Energy’ with the help ofsuitable examples and

� explain the term power and define its SI unit.

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13.1 WORK

Work is a common term we use in our day to day conversation. Ordinarily we includestanding, reading, lying etc. in the category of work. But in sciences physical workhas a very specific meaning, that is, work is said to be done when force is appliedon a body and the body moves through some distance in the direction of force. Toelaborate, it implies that:

� If a force is applied on a body and the body does not move then no work isdone at all.

Example: When you try to push a wall you do not do any work as distance movedby the wall is zero (Fig. 13.1).

� If no force is applied on a body and the body is either at rest or moving witha constant velocity then again no work is done.

Example: A car moving with a constant velocity on a level road does not do anynet work. Because the fuel it consumes is used in doing work against fraction, sothat, its velocity may be maintained.

� If the force and displacement are perpendicular to each other, the work doneby the force is zero as shown in Fig 13.3.

Fig. 13.1 No displacement, no work is done in case of pushing a wall

13.2 RELATION BETWEEN WORK, FORCE ANDDISPLACEMENT

Work is measured as the product of force and the displacement in the direction ofthe force.

i.e., work = force × displacement in the direction of the force.

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If force and displacement are in the same direction you can easily find work doneby finding their product. But if force and displacement are in different directions thework done is obtained by finding the product of force and the projection ofdisplacement in the direction of the force. For the situation shown in Fig. 13.2.

work done W = F × PR and not F × PQ

Fig. 13.2 Work done when force and displacement are in different directions

Example: A person carrying a heavy load on his head and moving on a level roaddoes no work against gravity, because, there is no component of displacement inthe direction of force of gravity as shown in Fig 13.3.

Fig. 13.3 No work done against gravity when a person moves on a levelroad carrying a heavy load on his head

The SI Unit of work is newton-metre (Nm) or joule (J). 1 J work is done whena body moves through a distance of 1m under a force of 1 N, in the direction offorce.


Choose the correct option

1. (i) Work done is zero:

(a) When force and displacement are in the same direction.

(b) When force and displacement of the body are in opposite directions

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(c) When force acting on the body is perpendicular to the direction of thedisplacement of the body.

(d) When force makes an angle with displacement

(ii) 1 J of work is done when a force of 0.01 N moves a body through a distanceof :

(a) 0.01 m (b) 0.1 m (c) 1 m (d) 10 m (e) 100 m

(iii) In which of the following situations work is done?

(a) A person is climbing up a stair case.(b) A satellite revolving around the earth in closed circular orbit(c) Two teams play a tug of war and both pull with equal force(d) A person is standing with heavy load on his head

2. A car of mass 500 kg is moving with a constant speed of 10 ms–1on a roughhorizontal road. Force expended by the engine of the car is 1000 N. Calculatework done in 10 s by:

(a) net force on the car (b) gravitational force(c) the engine (d) frictional force

13.3 ENERGY AND ITS RELATION WITH WORK

When you play for a long time or do a lot of physical work at your home or outsideyou get tired, i.e., your body shows unwillingness or reluctance towards further playor work. At this time you may also feel hungry. After taking rest for some time or/and eating some thing you may again be ready for work. How does one explain theseexperiences? In fact, when you do work, you spend energy and more energy isrequired to do more work. The capacity of a body to do work is determined bythe energy possessed by it.

i.e., Energy possessed by a body = Total work that the body can do

Energy has the same unit as work, i.e., joule denoted by J.

However, conversion of 100% of energy may not always be practicable, because,in the process of conversion of energy into work some energy may remain unusedor may be wasted. To understand this point perform the following activity.

ACTIVITY 13.1

Alok and Kapil are inflating long (at least 5 cm) balloons in different ways as shownin Fig. 13.4. Alok blows in his balloon with part of its opening ushering in air, whileKapil blows in air in the whole area of the opening.

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Fig. 13.4 Alok and Kapil are inflating identical long balloons in different ways

� Which of them is making more effort?

� Which of them is doing more work?

Do this activity, find out which technique resulted in a bigger baloon can you explainthe reason.

On the basis of your conclusion now you can understand why air is blown from adistance in Phukini (metal pipe) to light the fire in Chulhas.

Fig. 13.5 Use of Phukini (metal pipe) to light the fire

Note: This cooking practice is unhealthy as it can lead to several health relatedproblem.

13.4 DIFFERENT FORMS OF ENERGY

You do work by spending muscular energy which you gain from the chemical energyof the food you eat. Your fan runs on electrical energy. While playing with magnetsyou might have seen that a magnet can move a piece of iron so it has magnetic energy.Thus energy is available to us in many different forms like mechanical, thermal, light,electrical, magnetic, sound and nuclear. Let us acquaint ourselves with different formsof energy.

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1. Mechanical Energy

This is the capacity of doing work that a body possesses by virtue of its position(potential energy) or by virtue of its motion (kinetic energy).

(a) Potential Energy

A body (say hammer) raised to a certain height above the ground when left to itself,falls down. If it is allowed to fall on a piece of dried clay it may break it into pieces.A body raised above the ground has thus ability to do work i.e. it has energy. Thisenergy possessed by a body raised above the ground is called its potential energy.

When two bodies one lighter and another heavier are dropped from the same heighton a pit of sand it will be found that the heavier body penetrates more in sand thanthe lighter body. Hence a heavier body possesses more potential energy.

If same body is dropped from different heights, we find that the body dropped froma greater height penetrates more, hence it has more potential energy. Potential energyof a body, thus depends on

� Weight of the body (W = mg)

� Height of the body (h) above the ground

It is found that the relation between Potential energy PE (Ep), weight (W), and height(h) is Ep = W × h = mgh

(b) Kinetic Energy

Kinetic energy is the capacity of doing work that a body has by virtue of its motion.To understand the factors on which the kinetic energy of a moving body dependsperform the following activity.

ACTIVITY 13.2

Make a stack of two thick hard bound books (about 10 cm) as shown in Fig.13.4.Let a hard bound register be placed on it to form a sloping plane. Place a matchbox near the plane with its length parallel to the horizontal edge of the incline. Leta pencil cell roll down the incline and hit the match box. Does the match box move?

Yes. The rolling cell had some kinetic energy due to which it made the match boxmove through a distance. Thus a moving object has ability to do work.

Now placing the match box at the same position let a torch cell roll from the sameheight and strike the match box. Does it move again? Does it move through a longerdistance? Why does it do so? The torch cell has more mass than pencil cell so ithas more kinetic energy and does more work.

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300

Fig. 13.6 Experimental setup to demonstrate conversion of potentialenergy into kinetic energy

Now repeat the experiment by making the cell roll from a greater height. Does itmove the match box through still more distance? From these observations we mayconclude:

� When a body comes down from a height its potential energy decreases whereas its kinetic energy increases.

� The kinetic energy (KE) of a moving body depends on :

(i) its mass (m) – more the mass (for same velocity) more is its kinetic energy.

(ii) its velocity (v) – more the velocity (for same mass) more is its kineticenergy.

It is found that the kinetic energy of a moving body, K.E.= 21

2mv

2. Thermal Energy

This is a form of energy which flows into our body to give us sensation of hotnessand out of our body to give us sensation of coldness. You shall learn some moredetails about thermal energy in lesson 14.

3. Light Energy

The form of energy which enables us to see things is called light energy. You willstudy more about light energy in lesson 15.

4. Electrical Energy

You may be familiar with the energy that lights our bulbs, runs our fans, operatesour pumps, heats our rooms, turns on our TV and radio and runs the refrigeratorin our homes. The electrical energy is generated due to movement of chargedparticles. You will learn more about this form of energy in lesson 16.

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5. Magnetic Energy

You know that a magnet can attract a piece of iron. Thus magnets have an abilityto do work. The energy involved in the functioning of a magnet is called magneticenergy. You will study more about this form of energy in lesson 17.

6. Sound Energy

The form of energy which enables us to hear is called sound. Sound originates whena body vibrates giving out waves which travel to our ear through a material medium.You will study more about sound in lesson 18.

7. Nuclear Energy

The nuclear energy is a non-conventional form of energy which is released in nuclearreactions by conversion of mass into energy. You must have read in lesson 12 thatIndia is trying to generate electrical power through nuclear energy.


1. Explain the terms work and energy with one example each.

2. The ability to do work is called ……………

3. The SI unit of all forms of energy is ……………

4. Energy possessed by a spring is …………… energy.

5. The energy possessed by a body due to its position is called ……………energy.

6. The energy possessed by a body due to its motion is called …………… energy.

7. At height h the potential energy is Ep at height 2

h the potential energy would

be ……………

8. At height h the potential energy of a body of mass m is Ep. At the same height

the potential energy of a body of mass2

m would be ……………

9. A body of mass m moving with a speed v has kinetic energy, Ek. The bodyif moves with speed 2v, will have kinetic energy equal to ……………

10. A body of mass m moving with a speed v has kinetic energy Ek. A body ofmass 2m moving with the same speed will have a kinetic energy…………..

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13.5 ENERGY TRANSFORMATIONS ANDCONSERVATION

The various forms of energy mentioned in section 13.4 get converted from one formto another in different situations. This phenomenon of converting one form of energyto another form is called energy transformation. The following examples will makethe point clear.

� Potential energy of water stored in a dam changes into kinetic energy as waterfalls from a height. The kinetic energy of flowing water changes into kinetic energyof rotation of a turbine. The coil attached with the shaft of the turbine rotatesin a magnetic field to convert kinetic energy of rotation of the turbine into electricalenergy.

� In our homes an electric bulb (or tube light) converts electrical energy into lightenergy, electric oven (or heater or iron or soldering iron) convert electrical energyinto heat energy and electric pump (or motor) converts electrical energy intomechanical energy.

� An electric cell converts chemical energy into electrical energy; solar cellconverts light energy into electrical energy and a thermocouple changes heatenergy into electric energy.

� A microphone converts sound energy into electrical energy and a loudspeakerchanges electrical energy into sound energy.

� Heat engine converts heat energy into work (mechanical energy) and work doneagainst friction is converted into heat.

During transformation of energy from one form to another it remains constant. Thisis known as Law of Conservation of Energy.

(a) Photosynthesis (Solar energy (b) Bursting of fireworks (chemical→ chemical energy of food) energy → heat, light and sound energy)

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(c) Electric bulb (electrical energy (d) Loudspeaker (electrical energy→ light energy) → sound energy)

(e) Table fan (electrical energy (f) Physical exercise (chemical→ kinetic energy) energy of food → muscular energy)

Fig. 13.7 Some examples of energy transformation


Give one example each of the following energy transformations.

1. (i) Light energy into chemical energy.

(ii) Chemical energy into heat.

(iii) Chemical energy into electrical energy.

(iv) Mechanical energy into electrical energy.

(v) Thermal energy into electrical energy.

(vi) Light energy into electrical energy.

2. (i) A motor converts electrical energy into .....................

(ii) An electric heater converts electrical energy into .....................

(iii) A microphone converts sound energy into .....................

(iv) A loudspeaker converts sound energy into .....................

(v) A heat engine converts heat energy into .....................

(vi) When we rub our hands together we change work into .....................

Notes



304

13.6 POWER AND ITS UNIT

Have you ever heard statements such as : Quarter horse power motor is enoughfor the pump of a room cooler, one horse power motor will fill the tank in half thetime a half horse power motor does. Horse power is a unit of power. And whatis power? Power is a quantity which tells us how fast the work is done. Power isdefined as the time rate of doing work i.e., the amount of work done in unit time.

or Power =work done

time takenSI Unit of power is watt. One watt is the power spent when 1 J work is done in1 s. It is also measured in horse power. 1 horse power (H.P.) = 746 watts.

ACTIVITY 13.3

Move up a staircase slowly and then run up on it to the same height. In which casedo you get tired more? Why?

Your answer would be that it has to be more in the second case. Why so, because,in the second case you took lesser time and hence spent more power.

� About 1J of work is done when you take a glass of water (200 mL) fromyour dinning table to your lips – a distance of about half metre.

� A football player spends about 150 J of energy when he/she kicks a ball ofabout 1/2kg to a height of 3 m.

� A normal adult weighing about 50 kg does about 5000 J of work in ascendingup the staircase of a single storey building.

� In pulling out a 20 litre bucket of water from a 20 m deep well approximately4000 J of work is done.


1. Kamya climbs up a staircase in 5 minutes, Suraiya takes only 3 minutes in goingup the same staircase. The weight of Kamya is equal to the weight of Suraiya.

(i) Which of the two does more work?

(ii) Which of the two spends more power?

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2. Express 1.5 H.P. in SI Unit of power.

3. One cricket ball and One plastic ball are dropped from the same height. Whichwill reach the ground with

(a) more energy, (b) less power?


� Work is done when a force is applied on a body and the body has somedisplacement in the direction of the force.

� Work is defined as the product of force and the displacement in the directionof force.

� Ability to do work is called energy. The capacity of a body to do work isdetermined by the energy possessed by it.

� There are various forms of energy: mechanical, thermal, light, electrical, sound,magnetic and nuclear.

� Mechanical energy may be of two types: kinetic and potential.

� Energy can be changed from one form to another. The process is called energytransformation.

� During energy transformation energy is neither created nor destroyed. This factis due to the Law of Conservation of Energy.

� The rate of doing work is called power. SI unit of power is watt.

TERMINAL EXERCISE

1. Define the following terms and give their SI units. (a) Work (b) Power (c) Energy

2. List different forms of energy.

3. State Law of Conservation of Energy. Explain with the help of examples.

4. List the energy transformation taking place in a thermal power plant.

5. A ball of mass 0.5 kg has100 J of kinetic energy. What is the velocity of theball?

6. A body of mass 100 kg is lifted up by 10 m. Find

(a) The amount of work done.

(b) Potential energy of the body at that height (g =10 ms–2)

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306

7. Why road accidents at high speeds are much worse than the accidents at lowspeeds?

8. Two bodies of equal mass move with uniform velocities u and 4 u respectively.Find the ratio of their kinetic energies.

9. What would you like to prefer a ramp or a staircase to reach at the third floorof your hospital? Justify.


13.1

1. (i) c (ii) e (iii) a

2. (i) Zero (ii) Zero (iii) 105J (iv) –10+5 J

13.2

1. Work: Work is said to be done when force is applied on a body and the bodymoves through some distance in the direction of force. Example: A personclimbing up a staircase.

Energy: The ability to do work is called energy. Example: A weightlifter lifts theweight.

2. energy 3. joule 4. potential

5. potential 6. kinetic 7. Ep/2

8. Ep/2 9. 4Ek 10. 2Ek

13.3

1. (i) In photosynthesis green plants transform light energy into chemical energyof carbohydrates.

(ii) In digestion of food chemical energy of food is converted into heat.

(iii) Electrical cells convert chemical energy into electrical energy.

(iv) In electric generators mechanical energy is converted into electrical energy.

(v) In thermal power plants heat energy is converted into electrical energy.

(Note: a still better example would be a thermocouple which directlyconverts heat energy into electrical energy)

(vi) In Solar Cells Light energy is converted into electrical energy.

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2. (i) mechanical energy

(ii) heat energy

(iii) electrical energy

(iv) electrical energy

(v) mechanical energy

(vi) heat energy

13.4

1. (i) They both do work against gravity. Because both of them have equal weightand climb equal height they do equal work.

(ii) Because Suraiya takes lesser time in climbing up the staircase and poweris inversely proportional to time so, Suraiya spends more power.

2. SI unit of power is watt

and 1 H.P. = 746 watt

1.5 H.P. = 746 × 1.5 = 1119.0 watt = 1.12 kW

3 (a) Cricket ball (b) Plastic ball

Notes


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14

THERMAL ENERGY

In previous lesson, we have studied that one of the most common forms of energyis thermal energy. It is the energy due to which we feel hot or cold. If the energyflows into our body we feel hot and if it flows out of our body we feel cold. Toprevent heat from flowing out of our body we wear woolen clothes during winter.

Thermal energy is also called heat. We receive heat directly from the sun along withlight. The heat from the sun dries our clothes, ripens our crops and evaporates waterfrom water bodies to cause rain. We need heat to cook our food, to light the fire,to run a thermal power station. Generally, we produce heat for all such purposesby burning a fuel or by passing electric current through a conductor.

In antiquity, fire was produced by striking two stones together. We have now refinedthat method in the form of a match box. Heat is thus an important form of energy,connected intimately with our life and comfort.

In this lesson you will study about heat, its various effects and its role in our lives.

OBJECTIVES


� distinguish between heat and temperature;

� describe experiments to show the expansion in solids, liquids, and gases;

� describe the construction and working of a laboratory thermometer and aclinical thermometer;

� state different scales of temperature, viz .fahrenheit, celsius and kelvin;

� relate readings on fahrenheit, celsius, and kelvin scales of temperature andsolve numerical problems based on these relationships;

� give examples of latent heat and its applications in daily life and

• define specific heat and give its SI unit.

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14.1 HEAT AND TEMPERATURE

We know that thermal energy is provided to water in a kettle when it is placed onfire. If we touch water in the kettle before we start heating it and then after sometime of heating we find that the water becomes warmer. This degree of hotness orcoldness of a body due to which we call it warmer is called Temperature. Heat andtemperature are intimately related. Normally, more the heat given to a body higherwill become its temperature.

14.1.1 Heat

When water is boiled in a kettle the steam built up in the kettle raises its lid up andwhen the steam escapes out the lid falls down. Heat thus can do work, so, it is aform of energy. This property of steam was used to build steam engines – thedevices which convert heat of steam into mechanical work.

You may ask, is the converse operation also possible? Can we convert mechanicalwork into heat? Why not? Why don’t you recall that when you rub your handstogether they become warm? In fact work done against friction is always convertedinto heat.

The equivalence of work and heat was noticed and experimentally established byJ. P. Joule. While boring the barrel of a gun with a blunt borer Joule found that sohuge amount of heat was produced in the process that even water in which theprocess of boring was being carried out started boiling.

Through further experiments he found that one Calorie (Unit of heat prevalent atthat time) of heat is equivalent to 4.2 Joule of work.

14.1.2 Temperature

As discussed above temperature is a quantity which tells us how hot a body is? Ifa hot body is kept in contact with a colder body for some time, we will find thatthe hotter body does not remain that hot and the colder body becomes some whathotter. Thus heat is transferred from a hotter body (a body at higher temperature)to a Colder body (i.e. a body at lower temperature). Hence temperature is thedegree of hotness of a body which determines the direction of flow of heat.Heat always flows from a body at higher temperature to a body at lower temperature.

14.2 MEASUREMENT OF TEMPERATURE

You might have noticed that whenever a patient is brought to a doctor, the doctornormally measures his body temperature. Do you know the device the doctor usesto measure his body temperature? What do they call it? They call it thermometer.

There are different types of thermometers that they use for different purposes. Thethermometer that a doctor uses to measure the temperature of human body is called

Notes



310

Clinical thermometer Fig. 14.1(a). The thermometer that we use for measuringtemperature in science experiments is called laboratory thermometer Fig. 14.1(b)and the thermometer that the meteorologists use for determining the maximum andminimum temperature during a day is called as maximum – minimum thermometerFig. 14.1(c). These days they are using digital thermometers Fig. 14.1(d) fordifferent purposes.

(a) Clinical thermometer

(b) Laboratory thermometer

(c) Maximum – minimum thermometer

(d) Digital thermometer

Fig 14.1 Different types of Thermometers

Scale onCelsius

MinimumTemperatue

MercuryLevel

Scale onFahrenheit

U-Tube

MaximumTemperature

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Thermal Energy


14.3. CONSTRUCTION OF A THERMOMETER

Normally mercury-in-glass thermometer is conveniently used in day to day applications.In this type of thermometer there is a thin walled bulb attached to a thick walledcapillary. The bulb and to a certain height the capillary are filled with mercury byrepeated heating and cooling. The capillary above mercury level is evacuated andits upper end is sealed. Then the thermometer is calibrated (marked) to measuretemperature. For calibration lower and upper fixed points are marked respectivelyby burying the bulb first in melting ice and then in steam for sufficient time, so thatmercury level in the stem remains fixed with time in each case (Fig.14.2).

Fig. 14.2 Calibration of a thermometer

You may ask why use of mercury is preferred as thermometric liquid. The reasonsare many. Mercury acquires the temperature of the body, it is kept in contact withvery quickly; it absorbs very little heat from the body in contact and has large uniformexpansion over a wide range. It is opaque and does not stick to the walls of thecontainer. These properties make mercury the most appropriate liquid for accuratetemperature measurements over a wide range.

Giving different values to the lower fixed point and upper fixed point and dividingthe space between these two marks in equal number of divisions different scales aredeveloped for measuring temperature. Three such scales are shown in Fig. 14.3.These are: celsius scale, fahrenheit scale and kelvin scale. In celsius scale the lowerfixed point (ice point) is marked as 0, the upper fixed point(steam point) is marksas 100 and the intervening space is divided into 100 equal parts .In fahrenheit scalethe lower fixed point is marked as 32, upper fixed point as 212 and the interveningspace is divided into 180 equal pats. In case of a kelvin’s scale the lower fixed pointis marked as 273, steam point as 373 and the space between them is divided into100 equal parts. SI Unit of temperature is kelvin (K).

Ice point Steam point

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Fig. 14.3 Different scales of temperature

This is clear from Fig. 14.3 that the three scales are related by the formula

C

100=

F – 32 K–273 =

180 100(14.1)


State whether the following statements are true or false:

(i) Heat can be measured in kelvin.

(ii) –30° F is a lower temperature than –30° C.

(iii) The numerical value of temperature of any hot body measured on kelvin’s scaleis always higher than the value on Fahrenheit scale.

(iv) Thermal energy can be measured either in calories or in joules.

(v) Pure alcohol can also be used as thermometric liquid.

(vi) A body is felt cold when heat flows from our body to that body.

14.4 EFFECTS OF HEAT

When a body is heated changes may occur in some of its properties .These changesare the effects of heat. Some of the effects of heat, as you might have observed are:

14.1 Rise in temperature

When a body is heated its temperature increases, that is why, it appears warmerwhen touched.

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14.2 Change of state

When heat is supplied to a substance in solid state its temperature rises till at aparticular temperature it may change into its liquid state without any further changein its temperature. This characteristic constant temperature at which a solid changesinto its liquid state is called melting point of the solid. The melting point of a substanceis a characteristic, constant value and different substances may have different valuesof melting points (Table 14.1).

Conversion of a solid into its liquid state at its melting point is called change of statefrom solid to liquid (fusion) and the heat that is transferred to the substance duringmelting is called Latent Heat of Fusion. Because, it does not becomes apparentin the form of rise in temperature. Latent heat of fusion of a solid substance is definedas the amount of heat (in joules) required to convert 1kg of the substance from solidto liquid state at its melting point (Table 14.1).

Similarly, when heat is supplied to a substance in liquid state its temperature risesbut there is a possibility that it changes into its vapour state at a constant temperature.The heat supplied in this case is called Latent Heat of Vaporization. Latent heatof vaporization of a liquid is defined as the amount of heat (in joules) required toconvert 1kg of the substance from its liquid to gaseous state at a constant temperature.Latent heats of vaporization of different substances are also different (Table 14.1).

It may be noted that vaporization may take place in two different ways: (i)Evaporation from the surface of a liquid at any temperature (ii) Boiling of the wholemass of the liquid at a constant temperature called boiling point of the liquid. Boilingpoints of different liquids may also be different (Table 14.1).

Table 14.1 Melting, boiling points, latent heat of fusion and latentheat of vaporization of some materials

S. No. Name of Melting Latent heat Boiling Latent heat ofMaterial Point of fusion Point vaporization

(°C) (× 103 J/kg) (°C) (× 103 J/kg)

1. Helium –271 – -268 25.1

2. Hydrogen –259 58.6 –252 452

3. Air –212 23.0 –191 213

4. Mercury –39 11.7 357 272

5. Pure Water 0 335 100 2260

6. Aluminum 658 322 1800 –

7. Gold 1063 67 2500 –

This may again be noted that on cooling change of state may take place in reverseorder. The chart given below illustrates the various events of change of state.

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Fig. 14.4 Change of state

14.5 THERMAL EXPANSION

Take a metallic ring fitted with a handle and a sphere of the same metal fitted witha chain such that the sphere just passes through the ring (Fig. 14.5). Now heat thesphere in steam for some time and place itover the ring. Does it pass through the ringnow? It doesn’t. Obviously, the size of thesphere has increased on heating. In fact everymaterial (except water which contracts onheating from 0°C to 4°C) expands on heating.The increase in the size of a body on heatingis called thermal expansion.

The expansivity of different materials is normallydifferent. The fact can be easily noticed withthe help of a bimetallic strip. A bimetallic stripis a strip having two layers of two differentmetals one over the other. Consider thebimetallic strip made of steel and aluminium(Fig. 14.6). When we clamp one end of the strip and heat it uniformly with the helpof a Bunsen burner, it bends with aluminium layer outward. This clearly shows thataluminium has increased in length more than steel and caused bending.

Fig. 14.6 The bimetallic strip bends on heating

Fig. 14.5 Ball and ring experiment todemonstrate thermal expansion

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It can be seen that increase in length of a metallic bar will be more for a longer barand also for a greater rise in temperature of the same bar. Let us consider a metallicbar of length L0 at temperature 0° C. Increase in its length ΔL at a temperature Δtis given by:

ΔL ∝ L0Δt

ΔL = αL0Δt

α =0

L

L t

ΔΔ

Here α is a constant for the material of the bar and is called as the Linearexpansivity of the bar.

The Linear expansivity (or Coefficient of Linear expansion) of a material isdefined as the change in length per unit original length per degree celsius rise intemperature. The SI Unit of coefficient of expansion is per kelvin (which is sameas per degree celsius in magnitude).

A piece of solid may expand along length, breadth and height simultaneously hencethere will be an increase in its volume with temperature.

The Volume expansivity of a material may be defined as change in volume perunit original volume per degree celsius rise in temperature.

i. e. γ =V

V t

ΔΔ

The value of coefficient linear expansion (α) and the coefficient of volume expansion(γ) of some materials are given in Table 14.2.

Table14.2 Values of Coefficient of Linear expansion and Coefficientsof volume expansion of some common substances

S. No. Name of Coefficient of Linear Coefficient of VolumeMaterial Expansion (°C–1) Expansion (°C–1)

1 Quartz 0.4 × 10–6 1.2 × 10–6

2 Steel 8 × 10–6 24 × 10–6

3 Iron 11 × 10–6 33 × 10–6

4 Brass 18 × 10–6 54 × 10–6

5 Silver 18 × 10–6 54 × 10–6

6 Aluminium 25 × 10-6 75 × 10–6

7 Lead 2.9 × 10–6 8.7 × 10–6

The table clearly shows that expansivity of solids is very small therefore we cannotsee and measure expansion of solids easily. But liquids expand much more than solids

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316

and gases many times more than liquids and so we can see expansion of liquids andgases easily. However, since liquids and gases do not have a definite shape, it willbe volume expansivity only relevant for fluids.

ACTIVITY 14.1

Demonstration of expansion in liquids

Take a small transparent bottle (say an injectionbottle) fill it with water up to the brim. Make a smallhole in its cork and insert a thin transparent plasticpipe in it (say a used up empty ball-pan refill) sothat the lower end of the pipe dips in water andwater rises in the pipe up to a certain height. Markthe level of water in the pipe indicated as (A). Nowheat the bottle. What do you find? Does the levelof water in the pipe come down? Why so? Keepon heating the bottle. Does the level of water startincreasing after reaching a certain minimum level(B)? Does it shoot off the initial position (A) andrises further up to the height (C)? Why so? Canyou infer from this experiment that water expandsmore than glass for the same temperature rise?

ACTIVITY 14.2

Demonstration of expansion in gases

Take a thin walled narrow bored glass tube and entrap a drop of mercury in it. Thenheat one end of the tube and pressing it on a hardsurface seal this end. Let the tube cool to normaltemperature. Hold the tube vertically and markthe position of mercury in the tube. This way wehave entrapped a column of air between mercurydrop and sealed end of the pipe. Now even ifwe warm the air column by holding it in our handwe can see the drop of mercury shifting itsposition. Does it move up or down? What do youconclude from this experiment? Does this showthat gases have high expansion even for a smallrise in temperature?

Fig. 14.8 Expansion of gas

Fig. 14.7 Expansion of Liquid

Notes

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14.5.1 Uses of thermal expansion in day to day life

1. The property of thermal expansion is used in the construction of thermometers.

2. A tightly closed metallic cap of a bottle may be opened by using thermalexpansion. The cap on heating expands, becomes loose and may be openedeasily.

Fig. 14.9 Loosening a metal cap by heating it

3. Have you seen a horse-cart (Tanga)? It has wooden wheels on the rims of whichiron rings are mounted. Do you know how the iron ring is mounted on a woodenwheel? The iron ring is, in fact, made of a radius slightly less than the radius ofwheel. Then the ring is heated so that its radius becomes slightly more than theradius of the wheel. The ring is than slipped on the rim of the wheel while hot.Subsequently on cooling, it contracts and firmly holds the rim of the wheel.

Fig. 14.10 Fitting iron tyre on wooden wheel

4. Thermostats used in heating/cooling devices make use of a bimetallic strip toautomatically switch off the heating cooling circuit when the temperature rises/falls beyond a certain value. After some time when the temperature returns below/above a certain value the bimetallic strip resumes its original position and thecircuit again becomes on. A simple bimetallic thermostat is shown in Fig. 14.11.

Fig. 14.11 Principle of a thermostat

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318

5. We have to take care of thermal expansion while making big structures orotherwise these structures may collapse, for example:

(a) Gaps are left at the joints of a railway tracks [Fig. 14.12(a)] or else duringsummer due to thermal expansion the rails will bend and derail the train.

(b) The iron bridges are not made of continuous structures. At one end thegirders are left open and placed over rollers [Fig. 14.12(b)].

(a) (b)

Fig. 14.12 (a) Gap in railway tracks (b) Girders in iron bridges placed on rollers

6. While pouring hot tea in a glass tumbler it is suggested that a metallic spoon befirst placed in the tumbler and the tea be poured over it. In case the tea is directlypoured in the tumbler it may get cracked due to uneven expansion of its differentparts.


Fill in the blanks with the correct choice

1. A bimetallic strip is used as a thermostat in the electrical device named…………….. (electric bulb, T.V., refrigerator).

2. Melting point of 1 kg wax will be …………….. the melting point of 2 kg wax(double, half, same as).

3. Latent heat of evaporation is measured in …………….. (J, J/K, J/kg).

4. 1 kg steam at 100 °C has 2260 J …………….. heat than water at 100 °C(more, less).

5. The cubical expansivity of a substance is …………….. its linear expansivity(equal to, two times, three times)

6. The expansivity of …………….. is maximum. (solids, liquids, gases).

Notes

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14.6 SPECIFIC THERMAL CAPACITY OF A MATERIAL

When two bodies at different temperatures are kept in contact, heat is transferredfrom the hot body to the cold body till both of them acquire the same temperature.The two bodies then are called in thermal equilibrium. In acquiring thermalequilibrium the hot body loses heat and the cold body acquires an equal amount ofheat, i.e., heat lost by hot body = heat gained by cold body, provided we assurethat there is no loss of heat to the surrounding.

It can be seen that if the temperature of hot body is more, the rise in the temperatureof cold body will also be more i.e. heat transferred from a hot body to a cold bodyis directly proportional to their temperature difference,

Q ∝ ΔθSimilarly it can be shown that if the mass of cold body is more it will absorb moreheat from the cold body

i. e. Q ∝ m

so, Q ∝ mΔθ= msΔθ

Where s is a constant of proportionality which depends on the nature of the materialof the body. This is also called as the specific heat capacity of the material.

The specific heat capacity of a material is defined as the amount of heat (in Joule)required to raise the temperature of 1kg mass of that material through 1 K.

The SI Unit of specific heat capacity (or simply specific heat) is J kg–1 K–1

The specific heat capacities of different materials may have different values. Table4.3.3 gives the specific heat of some materials.

Table 14.3 Specific heats of some materials at 20°C

S. No. Substance Specific Heat Substance Specific Heat

J kg–1 Cal kg–1 J kg–1 K–1 Cal kg–1

K–1 K–1 × 103 K–1

1 Aluminium 875 0.29 Ethyl alcohol 2.436 0.58

2 Copper 380 0.091 Methyl alcohol 2.562 0.61

3 Caste Iron 500 0.119 Benzene 1.680 0.40

4 Wrought Iron 483 0.115 Ethene 2.352 0.56

5 Steel 470 0.112 Glycerin 2.478 0.59

6 Lead 130 0.031 Mercury 0.140 0.033

7 Brass 396 0.092 Turpentine 1.800 0.42

8 Ice 2100 0.502 Water 4.200 1.00

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320

From the table it is clear that of all substances water has highest value of specificheat.

Higher the value of specific heat of a substance lower will be the rate at which itis heated or cooled as compared to the substance of lower specific heat underidentical conditions.


Choose the correct alternative

1. Two iron balls of radii r and 2r are heated to the same temperature. They aredropped in two different ice boxes A and B respectively. The mass of ice melted

(a) will be same in the two boxes.

(b) in A will be twice than in B.

(c) in B will be twice than that in A.

(d) in B will be four times than that in A.

2. An iron ball A of mass 2 kg at temerature 20°C is kept in contact with anotheriron ball B of mass 1.0 kg at 20°C. The heat energy will flow

(a) from A to B only

(b) from B to A only

(c) in neither direction

(d) Initially from A to B and then from B to A.

3. When solid ice at 0°C is heated, its temperature

(a) rises (b) falls

(c) does not change until whole of it melts.

(d) first rises then falls back to 0°C.

4. When steam at 100°C is heated its temperature

(a) does not change. (b) increases

(c) decreases (c) none of these

5. Specific heat of aluminium is almost two times the specific heat of copper. Equalamount of heat is given to two pieces of equal masses of copper and ironrespectively. Rise in temperature of

(a) Copper will be equal to that of aluminium.

(b) Copper will be twice the rise in temperature of aluminium.

(c) Copper will be half the rise in temperature of aluminium.

(d) Copper will be four times the rise in temperature of aluminium.

Notes

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6. Equal heat is given to three pieces of copper A, B, and C having masses in theratio 1:2:3. The rise in temperature will be in the order

(a) A > B > C (b) B > C > A

(c) C > B > A (d) A > C > B


� Thermal energy is a form of energy and like any other form of energy can beused to do work. Therefore, the SI unit of thermal energy is also joule (J)

� Temperature is a measure of degree of hotness of a body and is measured indegree Fahrenheit (°F) or degree celsius (°C) or kelvin (K), with the help ofa thermometer.

� The three scales of temperature are related as C F – 32 K 273

100 180 100

−= =

� When change of state does not take place on heating a body its temperaturerises. The heat which does not become apparent in the form of rise in temperatureduring change of state is called latent heat.

� There are two types of latent heat (i) Latent heat of fusion of a solid. (ii) Latentheat of vaporization of a liquid

� The constant temperature at which a solid melts is called its melting point andthe constant temperature at which a liquid boils is called its boiling point. Meltingpoint and boiling point are characteristic properties of the substance.

� All substances expand on heating but different substances expand to differentextents when heated for same rise in temperature.

� Expansivity of a substance is a constant. Expansivity of different substances aredifferent.

� Liquids expand more than solids and gases expand very much more than liquids.

� Due to difference in expansivity of two metals a bimetallic strip bends on heating,This property of bimetallic strips is used in thermostats.

� Heat energy flows from a body at higher temperature to a body at lowertemperature till both of them acquire a common final temperature.

TERMINAL EXERCISE

1. Distinguish clearly between heat and temperature.

2. During change of state: (i) Is there a rise in temperature of the material on heatingit? and (ii)What happens to the heat we supply?

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322

3. Name the factors on which the thermal expansion of a wire depends.

4. Give any two uses of a bimetallic strip.

5. If you have a uncalibrated mercury thermometer how will you calibrate itinto a

(a) celsius thermometer (b) fahrenheit thermometer.

6. Explain the following:

(i) Why is mercury used as a thermometric liquid?

(ii) Why does a bimetallic strip bend on heating?

(iii) Why does steam at 100°C gives more severe burns than water at100°C?

(iv) Why do we use ice for cooling our drinks and not water at 0°C.?

7. Why is the heat given at the time of change of state called latent heat?

8. A certain amount of water is heated at a constant rate. The time to bring it toboiling is t1 and the time required from beginning of boiling to boiling off thewhole amount is t2. Which is greater t1or t2? Why?

9. At what temperature the numerical value of temperature on fahrenheit scale willbe double the value on celsius scale.

10. A 50 cm silver bar shortens by 1.0 mm when cooled. How much was it cooled?

(Given: Coefficient of linear expansion of silver = 18 × 10-6 per degree celsius)

11. How much heat energy is required to change 200 g of ice at –20°C to waterat 70°C?

(Given: Latent heat of fusion of ice = 335 kJ kg–1, and specific heat of ice =2100 J kg–1 °C–1, specific heat of water = 4.2 kJ kg–1 °C–1)


14.1

(i) False (ii) False (iii) True

(iv) True (v) True (vi) True

14. 2

1. refrigerator 2. same as 3. J/kg

4.. more 5. three times 6. gases

4.3

1. (d) 2. (c) 3. (c)

4. (b) 5. (b) 6. (a)

Notes

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15

LIGHT ENERGY

Light is the common form of energy. It makes the objects visible to us. You mighthave seen in torches there is curved sheet of metal around the bulb. Can you thinkwhy it is so? You may have also seen the stars twinkling in the sky in a clear night.Also on a clear day the sky appears blue at the time of sun rise or sun set whilesun near the horizon it appears orange or red.

Have you ever tried to find out the reason for such natural phenomenon? In this lessonyou will find the answer to all such questions. You will also study the defects of humaneyes and image formation in mirrors and lenses.

OBJECTIVES


� define reflection of light and state the laws of reflection;

� describe the image formation by plane and spherical mirror with suitableray diagrams in different cases;

� write mirror formula and define magnification;

� define refraction of light and state the laws of refraction;

� define refractive index of a medium and states its significance;

� give some examples in nature showing the refraction of light;

� describe various types of lenses and explain image formation by convex andconcave lens with the help of ray diagrams;

� write the lens formula and define magnification;

� explain power of lens and define diopter;

� explain the correction of defects of vision (near and far) by using lenses;

� explain how white light disperse through a prism and

� describe the scattering of light and give examples of its application in dailylife.

Notes


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15.1 REFLECTION OF LIGHT

Can you think how an object becomes visible to you? When we see an object wedo so because light from the object enters in our eyes. Some objects such as sun,stars, burning candles, lamp, etc. which emit light by their own are called luminousobjects. Some other objects maybounce back a part of the lightfalling on them from any luminousobject. This bouncing back of lightafter falling on any surface is calledreflection of light.

Thus, when a beam of light comesin contact with an object, a part orall of it gets bounced back. Thisphenomenon is called reflection oflight. Some objects having smooth and shiny surface reflect better than others. Asmooth shining surface, which reflects most of the light incident on it, is called amirror. In Fig. 15.1 reflection from a plane mirror is shown.

Greek mathematician Euclid explained how light isreflected. The phenomenon of reflection wastranslated into laws by an Arabian scientist Alhazanin about 1100 A.D.

To understand the phenomenon of reflection of light ray we define some terms. Thedirection of propagation of light, a beam of light consists of number of rays. Theincident ray is the ray of light falling on the reflecting surface. The normal is the linedrawn at 90° to the surface at the point where the incident ray strikes the surface.The light coming back from the reflecting surface is called reflected ray. The angleof incidence is the angle between incident ray and normal and angle of reflection isthe angle between reflected ray and normal.

15.1.1 Laws of reflection of light

Suppose a ray of light (IO) falls on a reflecting surface AB at O, after reflection itgoes along OR as shown in Fig. 15.2. The reflection of light from the surface takesplace according to the following two laws.

Alhazen (Ibn al-Haytham)(965-1040)

Fig. 15.1

Notes

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(i) Incident ray, reflected ray and the normal at the point of incidence, all lie in thesame plane.

(ii) The angle of incidence is equal to the angle of reflection i.e.,

∠i = ∠r

During reflection, there is no change in speed, frequency and wavelength of light.Reflection of light may be classified as regular reflection and diffused reflection.

15.1.2 Regular reflection

When reflecting surface is very smooth and the rays of light falling on it are reflectedstraight off it, then it is called regular reflection, as shown in Fig. 15.2.

Fig. 15.2 Regular reflection from a smooth plane surface

15.1.2 Diffused reflection

When the reflection of light takes place from rough surface the light is reflected offin all directions as shown in Fig. 15.3 is called diffused reflection.

Fig. 15.3 When surface is rough, parallel incident rays do not reflect parallel

In diffused reflection due to roughness of the surface normal drawn at the point ofincidence of parallel incident rays are not parallel, hence the reflected rays reflectin all direction but obey the laws of reflection.

15.2 FORMATION OF IMAGES DUE TO REFLECTION

You might have learnt that to see an object or image, the light from it should reachto the eyes of the observer. It means light coming from an object or image shouldfall on retina where from it will be sensed by brain with the help of optical nerves.

Notes



326

When light rays coming from the object meet or appear to meet at retina of eye,the object become visible and we say that the image of object is formed at retina.

When an object is placed infront of a mirror its image is formed by reflection. Everypoint on the object acts like a point source, from which a number of rays originate.In order to locate the image of the point object, an arbitrarily large number of raysemanating from the point object can be considered. However, for the sake ofsimplicity, we take any two rays of light (starting from the point object). The pathson reflection from the mirror (reflected rays corresponding to the incident rays) aretraced using laws of reflection. The point where these two rays actually meet is thereal image of the point object. If these rays appear to come from and not actuallycoming, the virtual image of the point is formed. Real images obtained by actualintersection of reflected rays, hence they can be projected on screen. Virtual imagesare obtained when the rays appear to meet each other but actually do not intersecteach other, hence they cannot be cast on screen.

ACTIVITY 15.1

Take a plane mirror on paper in vertical position. Use a pipe (straw) as incident beamat certain angle and coincide its image with another pipe (straw). You have to putthe second pipe in such a way that the image and this pipe remain in same line. Thesecond pipe (straw) will represent the reflected beam. Can you touch this image?Can you cut some part of this image by cutting the paper on which it is seen? Youcan not do it because the image formed is a virtual image.

Fig. 15.4

15.3 IMAGE FORMATION IN PLANE MIRROR

To understand the image formation in a plane mirror

(i) Put the mirror M1M2 in a vertical position over the sheet as shown in Fig. 15.5.

Image Plane mirror placedvertically

Eye

Objecti

r

Notes

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(ii) Put two pins, one at ‘A’ some distance away from the mirror and another onevery near to the mirror at ‘B’ so that, the line AB makes an angle with the lineM1M2 showing the position of the mirror.

(iii) Look at the images of A and B of the two pins through the mirror, put two otherpins at C and D so that all four pinsA, B, C and D are in the same straightline.

(iv) Now, look at the images of all thesepins closing one of your eyes andmoving your face side ways. If theimage of the two earlier pins and thetwo pins you have put just now appearto be moving together you can sayyour observation is free from parallaxerror.

(v) Join the positions of the pins by straightlines.

(vi) Keeping the first pin as it is, take out other three pins and repeat the experimentdescribed above by putting the pins in new positions. This way takes a fewmore reading.

To understand the formation of image, you may consider the light rays emerging outof the object A. We have drawn only three rays namely (a), (b), and (c). These raysafter striking the mirror M1M2 get reflected in the direction (d), (e) and (f),respectively, (as above shown in Fig. 15.5) obeying the laws of reflection.

It is clear that these reflected rays never meet with each other in reality. However,they appear to be coming emerging out from the point A′, inside the mirror i.e., ifthe reflected rays (d), (e) and (f) are extended in the backward direction, they willappear to meet with each other at A′. Thus at A′ we get the image of object A.

From the above activity we find that the image formed by a plane mirror has thefollowing characteristics.

� This image is virtual (i.e. it is not real), errect and the same in size as the object.

� The object distance and the image distance from the mirror are found to be equal.

i.e., OA = OA′

Hence, the image of a point in a plane mirror lies behind the mirror along the normalfrom the object, and is as for behind the mirror as the object is in front. It is an erectand virtual image of equal size.

Fig. 15.5 Image formation by a planemirror

Notes



328

15.3.1 A few facts about reflection

Put your left hand near a plane mirror. What do you see in the image formed byreflection? The image of your left hand appears as right hand of the image as shownin Fig. 15.6(a). Similarly, the number 2 will appear in an inverted fashion on reflectionas shown in Fig. 15.6 (b).Hence, due to reflection in a plane mirror left handedness is changed into righthandedness and vice-versa. This is known as lateral inversion. However, the mirrordoes not turn up and down. The reason for this is, that the mirror reverses forwardand back in three dimensions (and not left and right), i.e., only z-direction is reversedresulting in the change of left into right or vice-versa.For example a left handed screw will appear to be a right handed screw on reflectionas shown in Fig. 15.6 (c).

mirror mirror mirror

(a) (b) (c)

Fig. 15.6 Lateral inversion in image formed by a plane mirror

Similarly, if you read the sentence vki dk deky vki gh tkusaA in a mirror it willappear as

vki dk deky vki gh tkusaAIn a plane mirror the distance of the image is same as the distance of object fromthe mirror. If object distance from the mirror changes, the distance of image fromthe mirror will also change in the same way. It means if an object moves with velocityv towards the mirror, image will also move with same velocity v towards the mirrorand at every time the distances of the object and image from the mirror remain equal.However, the velocity of image towards the object will be 2v.

By drawing a ray diagram you conclude that you can see your full image in a planemirror whose height is half of your height. See the ray diagram in Fig. 15.7.

Fig. 15.7 Size of plane mirror to see the full image

Notes

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Think and Do

Take a L-shaped object and try to get the images as given below anddescribe the position of object in each case

A

B

C

D

Our eyes can notice the light of wavelength 400 nm (nanometre) to 70 nm. Thelight in this range of wavelength is called visible light. The light of wavelength morethan 700 nm (i.e., of red colour) is called infrared light and less than thewavelength of 400 nm (i.e., of violet colour is called ultra-violet light. All sourcesof light emit the combination of these three types of lights. Sun is a source whichemits very high percentage of visible light. In sun light 50% visible light, 40%infrared light and 10% ultra-violet light are present. Sun is the ultimate source ofall types of energy for us. Sun radiates 3.92 × 1026 joule of energy every second.Out of total energy radiated by sun about 0.0005% of energy reaches to earth.Earth receives 1.388 joule of energy per unit area every second from the sun.

The earlier fact about the nature of light was given by Pythagoras, a Greekphilosopher, in 6th century B.C. The objects are visible because of light travellingfrom eyes to the object and then back again. This theory could not stand the testof times and modified. This was due to the contributions of Newton (1642-1727)and Huygen (1670).

ACTIVITY 15.2

Place the following objects infront of a plane mirror and draw their correspondingimages in the given table.

Notes



330

Fig. 15.8

Table 15.1

Object Image

# .....................

O .....................

dke .....................

P .....................

OH .....................

Try to draw conclusion from this activity regarding image formation in a mirror.

ACTIVITY 15.3

Place a plane mirror at angle of 30°, 45°60° and 90° with horizontal. Now placean object (linear) in such a way that itsimage formed by plane mirror is alwaysstraight. Note down the angle made byobject with horizontal in the given table.

Table 15.2

Angle of mirror ααααα Angle of object θθθθθ

30° .....................

45° .....................

60° .....................

90° .....................


1. In column A, some sources of light are given. In column B, you have to writewhether these are lumineus or non-lumineus.

Source (A) Nature of source (B)

1. Glowing bulb 1. .....................2. Burning candle 2. .....................3. Moon 3. .....................4. Fire fly 4. .....................

5. Shining steel plate 5. .....................

Notes

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2. Write two differences between real and virtual image.

3. When you are standing infront of a plane mirror, a virtual and correct image ofyou is formed. If some one is taking a photograph of it using camera, what willbe the nature of image on photograph?

4. A light ray is falling on a plane mirror at 30° as shown in the diagram. If planemirror is rotated by 30° without changing the direction of incident ray, by whatangle the reflected ray will rotate?

Fig. 15.9

5. An object of height 10 cm is placed infront of a plane mirror of height 8 cm.What will be the height of image formed? Taking the distance of object fromthe mirror 6 cm, draw the ray diagram.

6. The image of an object placed at 10 cm from the mirror is formed at 10 cmbehind the mirror. If the object is displaced by 4 cm towards the mirror, by whatdistance will the image be displaced with respect to the (i) mirror (ii) object?

7. An object is moving with velocity 6 ms–1 towards a plane mirror, what will bethe velocity of image towards the (i) mirror (ii) object?

8. Some letters are given in following boxes. Make the meaningful words relatedto reflection of light choosing the horizontal and vertical sequencing.

N E P R E C T

O P X V R T U

R L V I R T U

M A L R E A L

A N I T C A R

L E O U T A E

A I M A G E J

N K N L E N C

9. The distance and height of an object placed infront of a plane mirror are givenin column A and B respectively. In column C and D the distance of image andheight of image are given but not in same order. Correct the order.

Notes



332

Distance of Height of Distance of Height ofobject (A) object (B) image (C) image (D)

10 cm 5 cm 10 cm 10 cm

5 cm 10 cm 5 cm 8 cm

6 cm 8 cm 6 cm 5 cm

15.4 REFLECTION AT SPHERICAL MIRRORS

A spherical mirror is a section of a hollow sphere whose inner or outer surface ispolished. Thus, there are mainly two types of spherical mirrors (i) convex mirror and(ii) concave mirror.

(i) Convex mirror: It is a mirror in which the reflection takes place from the bulgingsurface (i.e. inner side is painted and reflected surface is polished to make thesurface smooth as shown in Fig. 15.10.

(ii) Concave mirror: It is a mirror in which the reflection takes place from the caveside surface (i.e. outer side is painted and the inner or cave side surface ispolished to make the reflected surface smooth as shown in Fig. 15.10.

Fig. 15.10

To understand the reflection at spherical surface certain important terms are veryuseful. They are shown below in Fig. 15.11.

Fig. 15.11 Some important terms of spherical mirrors

Notes

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(i) Pole (P): It is the mid point of the spherical mirror. Point P is the pole inFig. 15.11.

(ii) Centre of curvature (C): It is the centre of a hollow sphere of which thespherical mirror is a part. It can be determined by finding the point of intersectionof two normal drawn at the spherical surface of the mirror. The point C is thecentre of curvature in Fig. 15.11.

(iiii) Radius of curvature (R): It is the distance between the pole and centre ofcurvature of the mirror. CF is the radius of curvature in Fig. 15.11.

(iv) Principal axis: It is an imaginary line joining the pole to the centre of curvature.Extended line CP is the principal axis in Fig. 15.11.

(v) Principal focus (F): The rays of light parallel and closed to the principal axisof the mirror after reflection, either pass through a point (in concave mirror)or appear to be coming from a point (in convex mirror) on the principal axis;this point is called principal focus of the mirror. Point F is the principal focusin Fig. 15.11.

(vi) Focal length (f): It is the distance between the pole and the principal focusof the mirror. PF is the focal length in the Fig. 15.11.

15.5 RELATIONSHIP BETWEEN FOCAL LENGTH ANDRADIUS OF CURVATURE

Consider the reflection of light of ray IM at M at a concave mirror. CM is the normaldrawn at the surface which passes through centre of curvature and MF is the reflectedray which passes through the focal point.

∠i = ∠r (as we know that angle of incidence and reflection are equal)

∴ in ΔCMF,

MF = CF

Fig. 15.12

Notes



334

For small aperture of the mirror,

MF = PF

⇒ PC = PF + CF = PF + PF = 2PF

R = 2f

where R = radius of curvature and f is the focal length of the mirror.

15.6 RULES OF IMAGE FORMATION BY SPHERICALMIRRORS

The ray diagram for image formation by mirrors can be drawn by taking any twoof the following rays. The point where these two rays meet or appear to be comingfrom the point will be the image point which determines the position of image.

(i) Ray striking the pole: The ray of light striking the pole of the mirror at anangle is reflected back at the same angle on the other side of the principal axis(Ray no 1 in Fig. 15.13).

(ii) Parallel ray: For concave mirror the ray parallel to the principal axis is reflectedin such a way that after reflection it passes through the principal focus. But fora convex mirror the parallel ray is so reflected that it appears to come fromprincipal focus. (Ray no. 2 in Fig. 15.13)

(iii) Ray through centre of curvature: A ray passing through the centre ofcurvature hits the mirror along the direction of the normal to the mirror at thatpoint and retraces its path after reflection (Ray no. 3 in Fig. 15.13)

(iv) Ray through focus: A ray of light heading lowards the focus or incident onthe mirror after passing through the focus returns parallel to the principal axis.

(a) (b)

Fig. 15.13 Image formation by spherical mirror (a) concave mirror(b) convex mirror

15.6.1 Formation of image by concave mirror

Using the above rules of image formation, the ray diagram for the image formed fordifferent positions of an object are given below.

Notes

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(a) When the object is (b) Object beyond C (c) Object at Csituated at ∞

(d) Object between (e) Object at F (f) Object between F and P

C and F

Fig. 15.14 Image formation by a concave mirror

15.6.2 Formation of image by convex mirror

Image formation in convex mirror is shown in Fig. 15.15.

Fig. 15.15 Image formation by a convex mirror

The position, nature and size of the image formed in concave mirror and convex mirrorcan be summarized as given in table below:

Table 15.3

Position of Position of Nature of image Size of imagethe object image formed

(A) For concave mirror

(i) between P and F behind the mirror virtual larger

(ii) at F at infinitely real highly enlarged

(iii) between F and 2F beyond 2F real larger

Notes



336

(iv) at 2F at 2F real same size

(v) beyond 2F between F and 2F real smaller in size

(vi) at infinity at F real highly diminished

(B) For convex mirror

anywhere between P and F virtual always smallerinfront of mirror

� Every part of a mirror may form a complete image of an extended object froma different angle and due to super-position of these images from different pointsfinal image is formed. The brightness of the image will depend on its lightreflecting area. Thus a large mirror gives a brighter image than a small one.This phenomenon was used in a popular hindi film’s shooting at theSheeshmahal of ‘Amer Fort’ in Jaipur (Rajasthan).

� Though every part of a mirror may form a complete image of an object, weusually see only that part of it from which light, after reflection from the mirrorreaches our eyes. That is why:

(i) to see the full image in a plane mirror a person requires a mirror of atleast half of his height.

(ii) to see complete image of the wall behind a person requires a mirror ofat least (1/3) of the height of the wall and the should be in the middleof wall and mirror.

� If two plane mirrors are placed inclined to each other at an angle θ, the numberof images of a point object formed

≈360

1°⎛ ⎞−⎜ ⎟θ⎝ ⎠

, if 360°⎛ ⎞

⎜ ⎟θ⎝ ⎠ is even integer

≈360°

θ if 360°⎛ ⎞

⎜ ⎟θ⎝ ⎠ is odd integer

For example, there are 5 images formed by two mirrors at 60° angle.

� Two mirrors inclined to each other at different angles may provide samenumber of images, e.g. for any value of θ between 90° and 120° the numberof maximum images formed is n = 3. This in turn implies that if θ is given, nis unique but if n is given, θ is not unique.

� The number of images seen may be different from the number of images formedand depends on the position of observer relative to object and mirrors e.g.,if θ = 120° maximum number of images formed will be 3 but number of imagesseen may be 1, 2 or 3 depending on the position of observer.

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15.6.3 Uses of mirrors

(i) Plane mirror is used

� in looking glasses,

� in construction of kaleidoscope, telescope, sextant, and periscope etc.,

� for seeing round the corners,

� as deflector of light etc..

(ii) Concave minor is used

� as a reflector in searchlight, head light of motor cars and projectors etc.,

� for converging solar radiation in solar cookers,

� in flood lights to obtain a divergent beam of light to illuminate buildings,

� in reflecting telescopes etc..

(iii) Convex mirror is used

� as a rear view mirror in motor cars, buses and scooters,

� as safety viewers at dangerous corners and on upper deck of doubledecker buses etc..

15.7 SIGN CONVENTION AND MIRROR FORMULA

To measure distances with respect to a curved mirror, following convention isfollowed:

(i) All distances are measured from the pole of the mirror.

(ii) The distances measured in the direction of incident light, are taken as positive.

(iii) The distances measured in opposite direction of incident light, are taken asnegative.

(iv) The distances above the principal axis are taken positive, whereas those belowit are taken as negative.

You have seen the image formation in concave mirror. When an object is placed at2f (centre of curvature) the image is formed at 2f. If f be the focal length of theconcave mirror, u distance of object and v the distance of image, then

u = –2f

and v = –2f

and f can be given as

1

f =1 1

2 2f f+

− −

Notes



338

or1

f =1 1

v u+

This is called mirror formula and it can also be verified for convex mirror. Use thisformula and justify the image formation given in image diagrams.

15.8 MAGNIFICATION IN SPHERICAL MIRRORS

Often we find that a spherical mirror can produce magnified image of an object. Theratio of the size of the image to the size of the object is called linear magnification.

i.e., linear magnification (M) =size of the image ( )

size of the object ( )

I v

O u=

where v = image distance from mirror, u = object distance from mirror.


1. An object is placed infront of a concave mirror as shown in the Fig. 15.16. Writethe position and nature of the image. What is the focal length of the mirror?

Fig. 15.16

2. In what condition, the image formed by concave mirror is virtual?

3. At what position will the reflected ray shown in Fig. 15.17 intersect the principalaxis beyond focus or before focus?

Fig. 15.17

A

B

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Fig. 15.18 Coin placed in a tumblerfilled with water

4. What type of image will be formed if an object is placed beyond centre ofcurvature infront of a concave mirror?

5. Find the position of the object placed infront of a concave mirror of focal length20 cm if image is formed at the distance of 30 cm from the mirror.

6. Write two uses of concave mirror.

7. Write the nature of image formed in convex mirror.

8. Find the position of the image formed in convex mirror of focal length 12 cmwhen object is placed at the distance of (i) 8 cm, (ii) 12 cm and (iii) 18 cm fromthe mirror.

9. Complete the following table with corresponding positions of object and imagein case of concave mirror.

Position of object Position of image

(i) at F (i) .....................

(ii) between F and 2F (ii) .....................

(iii) (iii) between F and 2F

(iv) (iv) beyond 2F

(v) beyond 2F (v) .....................

10. Write two uses of convex mirror.

11. Does concave mirror always converges the light rays?

12. Write the conditions to produce a magnified image in concave mirror.

15.9 REFRACTION OF LIGHT

Have you ever seen a coin placed at thebottom of a tumbler filled with water?The coin appears at smaller depth as itsactual depth. Why does it happen so?We see an image where the light raysmeet or at the point where light seems tobe coming from.

When light comes out from water, itbends due to which the coin appearsvertically displaced as shown in Fig.15.18. Does it always happen? No, itdoes happen only when light passes fromone medium to another obliquely. Thebending of light depends upon the densityof the medium.

Notes



340

When light passes from denser medium to rarer medium it bends away from thenormal. When it passes from rarer medium to denser medium it bends towards thenormal. This phenomenon of bending of light is called refraction of light.Refraction of light is shown in Fig. 15.19.

Angle ofrefraction

Angle ofincidence

Normali

r

(a) (b) (c)

Fig. 15.19 Refraction of light

In Fig. 15.19 (b) and (c) light deviate from its path but in Fig. 15.19 (a) it doesnot deviate from its path. Is it refraction or not? Certainly it is refraction, for normalincidence light rays do not deviate from their paths. During refraction the frequencyof the light remains unchanged but its wavelength changes hence the speed of lightalso changes.

ACTIVITY 15.4

To study the refraction of light place a glass slab on a dressing sheet fixed on a woodendrawing board, sketch a pencil boundary. Draw a line OC meeting the boundaryline obliquely. Fix the pins A and B on that line. Now look for these pins from theother side of the glass slab.

Take a pin and fix it on the sheet such that A, B and E are in a straight line. Nowfix another pin F such that it is in a straight line with pins A, B and E. Remove theslab and the pins.

Draw a line joining the points F and E to meet the boundary at D. The line ABCgives the direction of incident ray on the glass slab while the line DEF gives thedirection of emergent ray. The line CD gives the direction of refracted ray within theglass slab. Draw normal N1CN2 at C and N3DN4 at D to the boundaries. Now youcan conclude that the ray of light, when going from a rarer (air) to a denser (glass)medium, it bends towards the normal. Also, the ray of light when goes from denserto rarer medium it bends away from the normal.

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r

N2OA

C

N1

N4

D E

F

N3

Bi

Fig. 15.20 Refraction through a glass slab

15.9.1 Refractive Index of the Medium

When light travels from one medium to another its speed changes. A ray of light froma rarer medium to a denser medium slows down and bends towards the normal.On the other hand the ray of light going from a denser medium to a rarer mediumis speeded up and bends away from the normal. It shows that the speed of lightin different medium varies. Different s medium have different abilities to bend or refractlight. This bending ability of a medium is known as the index of refraction or refractiveindex. It is defined as the ratio of the speed of light in vacuum to that in the materialmedium.

Therefore, refractive index of a medium,

n ≈speed of light in vacuum

speed of light in medium

15.10 LAWS OF REFRACTION

The extent, to which a ray bends, depends not only on the refractive index of medium,but also on the angle of incidence. The laws of refraction are:

(i) First law of refraction: The incident ray, refracted ray and the normal at thepoint of incidence, all lie in the same plane (Fig. 15.19).

(ii) Second law of refraction: How much ray of light refracted depends on thatmedium. The ratio of the sine of the angle of incidence to the sine of the angleof refraction is constant and equal to the refractive index of that medium. Thislaw is also called Snell’s law.

Notes



342

Refractive index (n) ≈sine of angle of incidence

sine of angle of refraction

or n ≈sin

sin

i

r

Does the colour of light change during refraction?

The wavelength and frequency of light are related to the velocity as v = νλ, whereν is frequency and λ is wavelength.

ACTIVITY 15.5

Take a transparent bucket of plastic filled with water. Keep your head inside thewater in bucket and hold it above the red colour light bulb as shown in Fig. 15.21.What do you observe? Is there any change in the colour of light seen by you fromthe water? No, there is no change in the colour of light. It means when light goesfrom one medium to another, only its speed and wavelength change but the frequencyremains constant. It proves that colour is the function of frequency not the wavelengthof light.

Fig. 15.21 The red bulb is seen by a boy keeping his head insidethe bucket filled with water

15.11 REFRACTION THROUGH SPHERICAL SURFACE

In this section we will discuss refraction of light through a lens. A lens is a portionof a transparent refracting medium bounded by two surfaces. Depending upon thenature of surfaces lens may be of following types.

Notes

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Convex or Concave or Plano- plano- Convexo Concavodouble double convex concave concave convexconvex concave

Fig. 15.22 Different type of lenses

(i) Convex lens: Convex lens has its two surfaces bulging outward. It makes theparallel rays of light to converge to a point. Hence, it is called converging lens.The point of convergence is called focus as shown in Fig. 15.23.

Fig. 15.23 Converging action of a convex lens

(ii) Concave lens: A concave lens has its two surfaces caving inward as shownin Fig. 15.24. It makes parallel rays of light to spread from a point. Hence itcalled diverging lens. The point where from light rays appear to diverge iscalled focus as shown in Fig. 15.24.

Fig. 15.24 Diverging action of a concave lens

Notes



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15.12 IMAGE FORMATION IN LENSES

In order to draw the image formed by any lens, only two rays are required. Thesetwo rays are:

(i) A ray parallel to the principal axis of the lens converges after refraction at theprincipal focus of convex lens. It appears to diverge off in the case of concavelens.

(ii) A ray towards the optical centre falls on the lens symmetrically and afterrefraction passes through it undeviated.

The image formations in convex and concave lenses are shown in Fig. 15.25.

(a) Object is between focus and lens

(b) Object at the first focus

(c) Object is between F2 and 2F2

Notes

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(d) Object is at 2F2

(e) Object is beyond 2F1

(f) Object placed between optical centre and first focus

(g) Image by concave lens

Fig. 15.25 Image formation in convex and concave lens

Notes



346

All these images formed for different positions of object and nature of the image canbe summarized as given in the table below:

Table 15.4

Position of the Position of Nature of image Size of imageobject image formed

(A) For convex lens

(i) between F infront of lens virtual and enlargeand pole erect

(ii) at F at infinitely real and inverted highly enlarged

(iii) between F beyond 2F real and inverted enlargeand 2F

(iv) at 2F at 2F real and inverted same size

(v) beyond 2F between F real and inverted smaller in sizeand 2F

(vi) at infinity at F real and inverted highly diminished

(B) For concave lens

anywhere on the same virtual and always smallerinfront of lens side between F erect

and pole

15.13 SIGN CONVENTION AND LENS FORMULA

In case of spherical lenses,

(i) all distances in a lens are to be measured from optical centre of the lens

(ii) distances measured in the direction of incident ray are taken to be positive

(iii) distance opposite to the direction of incident ray are taken to be negative

(iv) the height of the object or image measured above the principal are taken positivewhereas below it, are taken negative.

Using the above mentioned sign convention and the image formation in Fig. 15.25let us assume, the distance of object from the optical centre of the lens to be u distanceof image from the optical centre to be v and focal length of the lens is f then therelationship between u, v and f for lens can be shown as:

1 1 1

f v u= −

This is called lens formula. Focal length for convex lens is positive, for concave lensit is taken negative.

Notes

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15.14 MAGNIFICATION

You would have notice that in case of some lenses, the size of the image of an objectis enlarged where as in some other cases it is diminished. If we take the ratio ofthe size of the image to the size of the object for a particular lens it remains constantfor that lens. The ratio of the size of the image to that of the object is called as themagnification of the lens.

Magnification =size of image ( )

size of object ( )

I

θ

or m =( )

( )

I

O

Also( )

( )

I

O =v

u

or m =v

u


1. Name the type of lens which always produces virtual image.

2. Draw the ray diagram for the image formation in convex lens where object isplaced at (i) F (ii) between F and 2F (iii) beyond 2F.

3. Draw the ray diagram for image formation in concave lens.

4. The sizes of the image and object are equal in a lens of focal length 20 cm. Namethe type of lens and distance of object from the lens.

5. An object of size 10 cm is placed infront of convex lens of focal length 20 cm.Find the size of the image formed.

15.15 DISPERSION OF LIGHT THROUGH GLASS PRISM

A prism is a transparent medium bounded by any number of surfaces in such a waythat the surface on which light is incident and the surface from which light emergesare plane and non-parallel. Generally equilateral, right angled isosceles or right angledprisms are used.

When white light or sun light passes through a prism it splits up into constituentcolours. This phenomenon is called dispersion and arises due to the fact thatrefractive index of prism is different for different colours of light. So, different colours

Notes



348

in passing through a prism are deviated through different angles. Rainbow, the mostcolourful phenomenon in nature, is primarily due to the dispersion of sunlight by raindrops suspended in air. Dispersion of light in glass prism is shown in Fig. 15.26.

Fig. 15.26 Dispersion of light

ACTIVITY 15.6

To produce a spectrum (display of different colour) using a prism and sunlight

(i) Take an empty card board box. Make a rectangular opening on its cover witha knife and close it with transparent while paper to see the spectrum.

(ii) Make a thin slit with knife on the opposite side of card board box.

(iii) Place the prism on a block inside the box.

(iv) Turn the slit-side face of the box towards sun light.

(v) See the coloured strips on the transparent paper.

The frequency of colours in decreasing order is violet, indigo, blue, green, yellow,orange, and red. It can be written as VIBGYOR.


1. When light passes from air to a medium its speed reduces to 40%. The velocityof light in air is 3 × 108 ms-1. What is refractive index of the medium?

2. When sunlight is passed through prism, it splits into seven colours as shown inFig. By numbers write corresponding colours.

White light

(Sun light)

r0

P

Q SGlass prism

Least bending Spectrum

Most bending

RedOrange

YellowGreenBlue

IndigoViolet

Notes

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Fig. 15.27

3. How do r and δ change for same angle of incidence i if the prism shown in Fig.is immersed in water

Fig. 15.28

4. Why does white light split into seven colours when it passes through a prism?

5. Write a natural phenomenon of dispersion of light.

15.16 EYE AND ITS DEFECTS

In eye a convex lens forms real, inverted and diminished image at the retina. Thelens can changes its convexity to form a suitable image as the distance between eyelens and retina fixed. The human eye is most sensitive to yellow-green light havingwavelength 5550 Å, the least to violet 4000 Å and red 7000 Å.

The size of an object as perceived by eye depends on its visual angle. When anobject is distant, its visual angle θ1 and image I1 at retina is small hence it will appearsmall. If it is brought near the eye, the visual angle θ° is large and hence size of imageI2 will increase as shown in Fig. 15.29.

The far and the near points for normal eye are usually taken to be at infinite and25 cm respectively. It means a normal eye can see very distant objects clearly butnear objects only if they are at a distance greater than 25 cm from the eye. The abilityof eye to see objects from infinite distance to 25 cm is called power ofaccommodation.

1234567

�

ir

Notes



350

Fig. 15.29 Image formation in eye

If an object is at infinity, i.e., parallel beam of light enters the eye, the eye is leaststrained and said to relaxed or unstrained. However, if the object is at the leastdistance of distinct vision (= 25 cm), eye is under the maximum strain and visual angleis maximum. (The angle made by object at eye is called visual angle).

Relaxed eye Maximum strained eyeFig. 15.30

If image of the object does not form at retina the eye has some defects of vision.Following are the common defects of vision.

(i) Myopia: In this defect the distant objects are not clearly visible i.e., far pointis at a distance lesser than infinity and hence image of distant object is formedbefore the retina as shown in Fig. 15.31. This defect is removed by usingdiverging (concave) lens. Myopia is also called short sightedness or nearsightedness.

Maypic eye Corrected eyeFig. 15.31

Notes

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Light Energy


(ii) Hyper metropia: It is also called long sightedness or far sightedness. In it thenear objects are not clearly visible i.e. near point is at a distance greater than25 cm. So the image of near object is formed behind the retina. This defectis removed by using converging lens as shown in Fig. 15.32.

Hypermetropic eye Corrected eyeFig. 15.32

(iii) Presbypia: In this defect both near and far object are not clearly visible i.e.,far point is lesser than infinity and near point greater than 25 cm. This can beremoved either by using two separate spectacles one for myopia and other forhypermetropia or by using bifocal lens. It is an old age disease. At old age ciliarymuscles lose their elasticity so they can not change the focal length of eye lenseffectively and eye losses its power of accommodation.

(iv) Astigmatism: It is due to imperfect spherical nature of eye lens. The focal lengthof eye lens is in two orthogonal directions become different so they can notsee objects in two orthogonal directions simultaneously. This defect in directioncan be removed by using cylindrical lens in a particular direction.


1. Identify the eye having defective vision from the following diagrams. Write thetype of defect in vision. How this defect can be removed?

(a) (b)

(c)Fig. 15.33

Notes



352

2. Three students Riya, Tiya and Jiya in a class are using sphericals of power +2D,+4D and –2D. What type of defect in vision they have?

3. How does the focal length of the eye changes when a lens is used to correctthe defect of vision in case of (i) short sightedness and (ii) long or for sightedness


� Light is a form of energy which makes the objects visible to us.

� When light falls on a smooth and rigid surface and comes back to the samemedium, the phenomenon is called reflection.

� In reflection, the angle of incidence is equal to the angle of reflection. Also theincident ray, reflected ray and normal drawn at the point of incidence all lie inthe same plane.

� In plane mirror, the virtual image of the size of object and at equal distance fromthe mirror is formed.

� Spherical mirrors are of two types (i) concave and (ii) convex.

� In spherical mirrors radius of curvature is double of the focal length

� When object is placed infront of a concave mirror at F, between F and 2F, at2F, beyond 2F, the image will be formed at infinity, beyond 2F, at 2F andbetween F and 2F respectively.

� When an object is placed between F and pole of the concave mirror, the imageis formed behind the mirror, virtual and enlarge in size.

� In convex mirror image is always formed between F and pole, smaller in sizeand virtual nature.

� When light goes from one medium to another its speed changes and the lightray bends. This phenomenon is called refraction of light.

� In refraction, the ratio of sine of angle of incidence to the sine of angle of refractionis constant called refractive index.

� When light goes from rarer to denser medium it bends towards the normal andangle refraction remains less than the angle of incidence.

� When light goes from denser to rarer medium it bends away from the normaland angle of refraction remains greater than the angle of incidence.

� A transparent medium bounded by two well defined surfaces is called lens. Thereare two types of lens (i) which converges light (convex lens) and (ii) whichdiverges light (concave lens)

Notes

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� In convex lens, when object is placed at F, between F and 2F, at 2F, beyond2F infront of convex lens the image is formed at infinity, beyond 2F, at 2F andbetween F and 2F respectively.

� When object is placed between F and optical centre of the convex lens, the imageformed is virtual and enlarge.

� In concave lens the image is always formed between F and pole, smaller in sizeand virtual.

� The focal length of a mirror f is given as:

1 1 1

f v u= +

� The focal length of a lens is given as:

1 1 1

f v u= −

� The reciprocal of the focal length is called power of the lens ( )1

Pf m

= . Its

unit is diopter.

� A person who can see the objects near to him properly but can not see the distantobjects has a near sight defect of vision. This defect can be removed by usinga concave lens.

� A person who can see the far objects but can not see the near objects has afar sight defect of vision. This defect can be removed by using convex lens.

� When light passes through a prism it splits into its constituent colours and thisphenomenon is called dispersion of light.

� Rainbow is the best known example of dispersion in nature.

TERMINAL EXERCISE

1. What happens to the speed of light when it goes from (i) denser medium to rarermedium (ii) rarer medium to denser medium?

2. Can angle of incidence be equal to the angle refraction? Justify.

3. Does a convex lens always converge light? Explain.

4. Write the nature of the image formed by concave lens.

5. In horizontal and vertical boxes of the letter grid some meaningful words regardingthe properties of light are placed in different rows and column in the table below.Find at least three and define them?

Notes



354

C O A C O N C A V E C Z

C O N V E X E W I M C W

V L R E F L R C T I O N

I O E I S E R T A R N P

R T F M A N E C A R C Y

T A R A T S C T E O A X

U M A G N E T O P R V W

A C C E P Q R S T U E V

L O T P R I M E T I M E

C V I K T U A L M G I N

A C O V E R T E X A R P

P N U M I R R O R R S Q

6. What will be the nature of the image formed in a convex mirror and in a concavemirror each of focal length 20 cm and object is placed at the distance of 10 cm.

7. Find the position of the image formed in concave mirror of focal length 12 cmwhen object is placed 20 cm away from the mirror. Also find magnification.

8. In which of the following media, the speed of light is maximum and in which itis minimum.

Medium Refractive index

A 1.6B 1.3C 1.5D 1.4

9. The image of a candle formed by a convex lens is obtained on a screen. Willfull size of the image be obtained if the lower half of the lens is printed blackand completely opaque? Illustrate your answer with a ray diagram.

10. Can a single lens ever form a real and errect image?

11. What is dispersion of light? What is the cause of dispersion of light?

12. Why do distant object appear to be smaller and closer to each other?

13. A person looking at a net of crossed wires is able to see the vertical directionmore distinctly than the horizontal wires. What is the defect due to? How is suchdefect of vision corrected?

Notes

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14. A person can see the objects placed at a distance of 30 cm clearly but cannotsee the objects placed 30 m away. What type of defect of vision he has? Howis this defect of vision corrected?

15. Distinguish visible, ultraviolet and infrared light.

16. Which of the following quantities remains constant during reflection of light?

(i) speed of light

(ii) frequency of light

(iii) wavelength of light

17. Write the value of angle of reflection at both the reflecting surfaces M1 and M2held perpendicular to each other as shown in Fig. 15.34

Fig. 15.34

18. An object is placed infront of a plane mirror. The mirror is moved away fromthe object with the speed of 0.25 ms–1. What is the speed of the image withrespect to the mirror and with respect to the object?

19. Size of the image in a plane mirror of height 12 cm is 20 cm. What is the sizeof the object?


15.1

1. 1. Luminous 2. Luminous 3. Non- luminous

4. Luminous 5. Non- luminous

2. (i) Real image can be taken on screen while virtual can not.

(ii) Real image is formed due to light rays meeting at the screen. While virtualimage is formed due to light rays appear to meet at the screen.

3. Real

4. 60°

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356

5.

6. (i) 4 cm (ii) 8 cm

7. (i) 6.0 ms–1 (ii) 12.0 ms–1

8. Real, Erect, Plane, Virtual, Image

9.

Distance of Height of Distance of Height of object (A) object (B) image (C) image (D)

10 cm 5 cm 10 cm 5 cm

5 cm 10 cm 5 cm 10 cm

6 cm 8 cm 6 cm 8 cm

15.2

1. Position is equal to –8.55 cm, the image is real of focal length 5 cm

2. When object is between focal point and pole of the mirror.

3. before focus

4. Real, smaller in size and inverted

5. 60 cm infront of mirror

6. Saving mirror, magnifying mirror for dentist

7. Always virtual and smaller in size

8. (i) 4.8 cm (ii) 6 cm (iii) 7.2 cm

9. Position of object Position of image

(i) at F (i) at infinity

(ii) between F and 2F (ii) beyond 2F

(iii) beyond 2F (iii) between F and 2F

(iv) between F and 2F (iv) beyond 2F

(v) beyond 2F (v) between F and 2F

10. (i) in vehicle for rear view (ii) as safety viewers at dangerous corners

11. No, not always

Notes

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12. Object must be placed either between focal point and pole for virtual image orbetween F and 2F for real image.

15.3

1. Concave lens

2. (i)

(ii)

(iii)

3.

Notes



358

4. Convex lens, 40 cm

5. –20 cm

15.4

1. 5/3

2. (1) Violet (2) Indigo (3) Blue (4) Green (5) Yellow

(6) Orange (7) Red

3. r and δ both will decrease

4. Material of the prism has different value of refractive index for different coloursof light.

5. Rainbow in the sky

15.5

1. (A) Shortsightedness, it can be removed by using diverging lens.

(B) No defect

(C) Longsightedness, it can be removed by using converging lens.

2. Riya and Tiya have longsightedness and Jiya has shortsightedness.

3. (i) increases (ii) decreases

Notes

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Electrical Energy


16

ELECTRICAL ENERGY

All of us have the experience of seeing lightning in the sky during thunderstorm. Wealso have experience of seeing a spark or hearing a crackle when we take off oursynthetic clothes in dry weather. This is Static Electricity. In your toys the sourceof energy is a battery in which chemical or some other energy is converted intoElectrical Energy. This electrical energy also comes from electrical power station toyour house through various devices and puts all comforts at our command just withthe press of a button. It provides us with heat and light. It powers big machines,appliances and tools at home and in industries e.g. ,radio set, computers, television,vacuum cleaners, washing machines, mixer and grinders, x-ray machines, electrictrains etc. Nowadays, it is impossible to think of a world devoid of electrical energy.Life without electricity even for short duration gives a feeling like a fish out of water.Here in this lesson we shall study the nature of electricity and way of its working.

OBJECTIVES


� cite examples of static electricity from everyday life;

� identify two kinds of electric charges and describe the Coulomb’s law;

� define the terms electrostatic potential, and potential difference;

� define electric current;

� state ohm’s law and define electrical resistance of a conductor;

� compute equivalent resistance of a number of series and parallel combinationof resistors;

� appreciate the heating effect of current by citing examples from everydaylife and

� define the unit of electric power and electric energy in commercial use andsolve problems about these.

Notes


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16.1 ELECTROSTATICSYou must have observed that a plastic comb when brought near a piece of paperdoes not pick up small pieces of a paper. But if you comb your dry hair and bringthe comb close to a small piece of paper, you will notice that the bits of paper areattracted towards the comb. Do you know why this happens? This happens becausethe comb gets charged or electrified when you comb your dry hair . The electricity(or charge) developed on a body on rubbing with another body is called frictionalelectricity or static electricity. Let us understand more with some simple activities.

An understanding of electric charge and their properties and also of magnetismbegan in 6th century B.C. i.e. 2500 years ago. One of the founders of Greekscience, Thales of Miletus knew that if a piece of amber is rubbed with a woolencloth, it would then attract light feathers, dust, lint, pieces of leaves etc. Amberis a yellow resinous (gum like) substance found on the shores of the Baltic sea.The Greek name for amber was ‘electrum’ which is the origin of the familiar wordselectricity, electric charge, electric force and the electron. However, the systematicstudy of electricity was done by Dr. William Gilbert, the personal physician ofQueen Elizabeth–1 of England. Dr. Gilbert had done the experiments i.e. therubbing of glass rod with silk, rubber shoes against a wooden carpet etc. whichproduced electrically charged bodies. Dr. Gilbert named amber like substancesElectrica, which became electrically charged by rubbing.

ACTIVITY 16.1

One day Dolly and Jolly were studying, suddenly Dolly spread some bits of paperon the table and asked her sister Jolly to lift the bits of paper with the help of a penor a balloon. Jolly brought pen near the bits of paper but there was no effect onbits of papers. Then she tried with balloon but could not show the magic. Jollyrequested Dolly to show the magic. Dolly took the pen and muttered somethingmeanwhile rubbing it on her sweater, she brought the pen near the pieces of paperand they got attracted towards the pen .This activity thrilled Jolly and she ran to tellthis to her mother. Similarly she rubbed an inflated balloon on her dry hair broughtnear the bits of paper, the pieces of paper got attracted towards the balloon. NowDolly rolled the pen between the palms of her both hands and then brought it nearthe bits of paper, the pen could not attract the bits of paper. Jolly was wonderingthat the trick was indeed some magic or some science was involved! Dolly explainedthat rubbed pen/inflated balloon attract bits of paper whereas before rubbing it doesnot attract bits of paper. After rolling between the hands, pen loses the property ofattraction. Hence, it is concluded that some bodies acquire electric charge on rubbingbut if it is touched to a conducting body in contact with ground, the charge leaksaway to the earth.

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It was realized that metal can be charged by rubbing but only if it is held in a handleof glass or amber. The metals cannot be charged if it is held directly in the hand.This is because electric charges move along the metal and pass through the humanbody (conductor) to the earth.

ACTIVITY 16.2

Take two straws (a hollow tube through which liquid is sucked), a small piece ofpaper, a piece of silk cloth, two pieces of threads (~50 cm), one small glass bottlea piece of cello-tape, scissors.

Take one straw and tie one thread at its centre and suspend it from the edge of atable with the help of a piece of cello tape so that it stays horizontally. Let it cometo rest. Now bring the other straw nearby the suspended straw and observe the effect.You will notice that there is no effect.

Now rub the suspended straw with a piece of paper and bring the other straw closeto one end of the suspended straw. Observe carefully the position of suspended straw.You will observe that the suspended straw moves towards the straw in your hand.

Rub the second straw (which is in your hand) with the piece of a paper and bringit close to one end of the suspended straw. Observe carefully the interaction betweenthe straws. The suspended straw moves away i.e. repelled away.

Now take the glass bottle and rub it with a piece of silk cloth and bring it close toone end of the suspended straw. Observe carefully the interaction between the strawand the glass bottle, the glass bottle attracts the suspended straw.

What do you infer? It is inferred that two uncharged straws do not affect each other.

(i) (ii) (iii) (iv)

(v) (vi) (vii) (viii)

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362

We observed that the charged straws repel each other but a charged straw and aglass bottle attract each other. Therefore it is concluded that:

(i) Two different types of charges (positive and negative) are produced.

(ii) Charge developed on glass bottle on rubbing it with silk cloth has a differentnature than the charge developed on straw rubbed with paper. From the basicexperiment it is established that glass on rubbing with silk cloth gets positivecharge which is opposite in nature to the charge acquired by the straw.

(iii) Like charges repel each other while unlike charges attract each other.

16.1.1 Nature of Charges

Have you ever experienced a shock when you touch a metal door knob after walkingacross a carpet? Let us try to understand this.

When we walk on a carpet made of insulating material such as rubber, nylon, woolor polyester, friction between soles of our footwear and the material of the carpetcause opposite charges to appear on them. When we touch the metal knob, the freecharge on our body(generated due to friction) and free charge on the ground causea discharge at a high voltage (several thousand volts to as much as 15,000 volts).

In early days a French chemist Charles Dufay observed that the charge acquiredby a glass rod rubbed with silk is different from the charge acquired by an eboniterod rubbed with fur/wool. Dufay termed the charge acquired by glass rod in firstcase as ‘vitreous’ and the charge acquired by ebonite rod on rubbing it with woolas ‘resinous’. Later on American scientist statesman Benjamin Franklin (1706-1790)introduced the terms positive in place of vitreous and negative in place of resinous,which is followed even today.

On rubbing, two materials acquire positive and negative charges equal in magnitude.Infact the process of rubbing does not create electric charges. It results in only transferof negative charges from one material to the other. The material, from which thenegative charges have been transferred, gets an excess of positive charge and theone which receives the negative charge becomes negatively charged. To answer thiswe have earlier studied that matter is made up of molecules and atoms. An unchargedbody contains a large number of atoms each of which contains an equal number ofprotons and electrons. In some materials some of the electrons are bound ratherloosely with their atoms. On rubbing, if some of the electrons are removed, thematerial which loses the electrons becomes positively charged and the material whichhas gained electrons becomes negatively charged. In the process of charging, positivecharges in atoms are firmly bound and do not participate in the process of charging.Conservation of charge states that the total amount of electric charge in an isolatedsystem (where no charge can get into or out of the system) does not change withtime. Within an isolated system interactions between different bodies of the system

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can cause transfer of charge from one body to another but the total amount of chargeof the isolated system always remains constant.

The Coulomb’s Law governs the force between the charged particles. It was firststudied by a French physicist Charles Augustine de Coulomb. Coulomb presentedthe inference of his experiments in the form of a lawwhich is called Coulomb’s law. According to Coulomb’slaw, the magnitude of the force of attraction (orrepulsion) between two point charges is directlyproportional to the product of the quantity of twocharges and inversely proportional to the square ofthe distance between them.

If a charge, q1 is placed at a distance, r from a similarcharge q2 the two charges will continue to repel eachother with a force

F =1 22

kq q

r

Where k is a constant of proportionality depending upon the nature of the mediumin which the charges are placed. In SI unit k = 9 × 109 N m2 C–2 for vacuum (orair). Charge is a scalar quantity. Coulomb is a SI unit of charge represented by C.

Fig 16.1 Two charges separated by distance r

If q1 = q2 = 1C, r = 1m

F =9 2 2

2

9 10 Nm C 1C 1C

(1m)

−× × × = 9 × 10–9 N

Thus, 1C is the charge when placed at a distance of 1m from an equal like chargein vacuum, experiences a repulsive force of 1N. Force is directed along the line joiningthe centres of the two charges. For like charges force is repulsive (positive in sign),while for unlike charges it is attractive (with negative sign).

16.2 ELECTROSTATIC POTENTIAL AND POTENTIALDIFFERENCE

Consider an uncharged body like a glass rod which is given a certain charge (saya positive charge), the body acquires that charge. Now if you wish to add morecharge of the same nature on it, the charge will experience a force of repulsion dueto already existing charge on it. Therefore, some work has to be done by any external

Coulomb (1736-1806)

Notes



364

agent to overcome this force of repulsion. This work will be stored up as electrostaticpotential energy in the system of charges. This is analogous to the process of raisinga body above the ground against the force of attraction in which work done againstgravity is stored in the body as its gravitational potential energy. Let a charge q bemoved upto a distance r towards a source charge Q, the electrostatic potential energypossessed by charge q is given by,

U =kQq

rThe electrostatic potential (or potential) at any point in the vicinity of a charge isdefined as the amount of work done in bringing a unit positive charge from infinityto that point. If W is the work done in bringing a positive charge q from infinity toa point in the vicinity of source charge Q, the potential V at the point due to chargeQ is

V =W

q orU kQ

q r=

Electrostatic potential is a scalar quantity (It has only magnitude and no direction).Its SI unit is joule/coulomb (JC–1) or volt (V) which is given in the honour ofAlessandro Volta (1745-1827) an Italian Physicist.

The potential at a point is 1 V if +1 C charge placed at that point possesses a potentialenergy of 1 J or the potential at a point is 1 V if 1 J of work is done in bringing1 C of positive charge from infinity to that point i.e.

1 volt =1 joule

1coulomb

Consider a charge q is placed at a point as shown in the

Fig. 16.2 charge q coming from infinity to B or C

Let B and C be two points where point B is closer to q than C. If a charge q isbrought from infinity to C or from infinity to B work done respectively be WC and

WB. The potential at points B and C respectively be BB

WV

q= and C

CW

Vq

=

The potential difference is the difference in potentials VB and VC. i.e.

VB – VC =B CW W

q

−

Where WB – WC is the work done in carrying charge from point C to B.

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Thus potential difference between two points B and C is equal to the amount of workdone in moving a unit charge from point C to point B.

Let us represent VB – VC as V; WB – WC as W the potential difference

V =Work done ( )

Amount of charge transferred ( )

W

q

The potential difference (pd) between two points of a conductor is said to be 1voltif 1 joule of work is done in moving 1 coulomb of charge from one point to another.Potential difference is a scalar quantity. It is measured using an instrument voltmeter.Voltmeter is always connected in parallel across which we have to measure thepotential difference.

Fig. 16.3 Voltmeter

Example 16.1: How many electrons make one coulomb?

Solution: Let n electrons make 1C (Since charge is built by the excess or deficiencyof electrons only).

Charge on 1 electron is 1.6 × 10–19C

Charge q = + n e

n =18

19

16.25 10

1.6 10

q

e −= = ××

electrons

Example 16.2: Calculate the work done in moving a charge of 3C across two pointshaving a potential difference of 24V.

Solution: Given q = 3C, V = 24V, W = ?

W = qV

= 3C × 24 V

W = 72 J

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366


1. Define the units of (i) charge (ii) electric potential.

2. When a glass rod is rubbed with a piece of silk it acquires +10 micro coulombof charge. How many electrons have been transferred from glass to silk?

3. How will the force between two small electrified objects vary if the charge oneach of the two particles is doubled and separation is halved?

4. How does the force between two small charged spheres change if their separationis doubled?

5. A particle carrying a charge of 1 micro coulomb (μC) is placed at a distanceof 50 cm from a fixed charge where it has a potential energy of 10 J. Calculate

(i) the electric potential at the position of the particle

(ii) the value of the fixed charge.

6. Two metallic spheres A and B mounted on two insulated stands as shown in theFig. 16.4 are given some positive and negative charges respectively. If both thespheres are connected by a metallic wire, what will happen?

Fig 16.4 Two metallic spheres mounted on stands

16.3 ELECTRIC CURRENT

All electrical appliances/gadgets like a bulb or a heater’s coil are based on themovement of charges as we know that flowing water constitute water current in rivers,similarly electric charge flowing through a conductor/a metallic wire constituteselectric current i.e., the quantity of charge flowing per unit time. Thus electric currentis the charge flowing through any cross section of the conductor in a unit time i.e.,

i = charge (Q)/time(t)

Where Q is the charge in coulomb flowing through the conductor in t seconds. If1 coulomb (C) of charge flows through any cross section of a conductor in 1 second(s), the current flowing it will be 1 ampere (A) i.e.,

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1 A = 1C/1s

Here, ampere is the SI unit of current given in thehonour of the French scientist Andre Marie Ampere(1775-1836). However, small currents are moreconveniently expressed in milliampere symbolicallyrepresented by mA, and microampere symbolicallyrepresented by μA. Current is a scalar quantity.

1 mA = 10–3A

1 μA = 10–6 A

An ammeter is an instrument which on connecting in series in an electrical circuitindicates how many amperes of current is flowing in the electric circuit.

Fig. 16.5 Ammeter

All metals contain large number of free electrons (~10–29 m–3) which act as chargecarriers. In a metallic conductor/wire these free electrons move with a sufficientlyhigh velocity of the order of 105 m s–1 in all possible directions between the atomsof the conductor/wire and even then there is no net flow of electrons. But whenbattery is connected across the ends of the conductor/wire, the electrons driftin one direction i.e., current flows along the wire in one direction from positiveterminal of the battery to the negative terminal of the battery along the wire witha very small velocity ~10-4m s–1 called drift velocity of the electrons.

We have already read that matter is made up of protons, electrons and neutrons.Protons carry positive charge, electrons carry negative charge and neutrons do notcarry any charge. An atom is electrically neutral but if a body carries excess of protons

Andre-Marie Ampere(1775-1836)

Notes



368

than the electrons, the body gets positively charge. If the body has excess of electronsthan the protons, body gets negatively charged. If a charged body is connected toan uncharged body through a metallic wire, the positive charge flows from higherpotential to lower potential while negative charge flows from lower potential to higherpotential. The charge flows till both the bodies are at the same potential. To passthe charge continuously from one body to another body through a wire a constantpotential difference has to be maintained between the two ends of a wire in a circuit.This is done by an external source of energy which forces the charge carriers(electrons) already present in the wire to move in a definite direction i.e. from lowerpotential region to higher potential region. The external source of energy is calleda cell. A cell is a device in which chemical energy is converted into electricalenergy. In the cell negatively charged plate repels the electrons which causes theelectrons to move along the wire. Hence the electrons flow from the negativelycharged plate through the wire to positively charged plate of the cell. This is knownas the electron current. Conventionally the direction of the current is taken as oppositeto the direction of the flow of electrons i.e., from the positive to the negative terminal.

Flow of electron/current

The combination of cells is called a battery. Oneof the earliest and simplest devices capable ofproducing steady current was invented by AlessandroVolta (1745-1827) named Voltaic Cell. Batteries area good source of Direct current. Direct current (DC)means the electric current is flowing in one directiononly in a circuit. To measure the current in a circuit,ammeter is used.

Caution: Never connect the two ends of a battery with conducting wire withoutmaking the electrons to pass through some load like a light bulb which slows theflow of current. If the electrons flow is increased too much, the conductor maybecome hot, and the bulb and the battery may be damaged.

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16.3.1 Conductors and Insulators

All materials can be divided into two categories on the basis of movement of chargesthrough them viz conductors and insulators.

Conductors are the materials which allow the electric current to flow through themquite freely e.g. metals like silver, copper, aluminum.

Insulators are the materials which do not allow electricity to flow through them freely.e. g. rubber, glass, bakelite etc..

16.3.2 Resistors

The electrical resistance is the tendency to resist the flow of electric current. A wirehaving a desired resistance for use in an electric circuit is called a resistor. It isrepresented by the symbol .

Resistance can be both either desirable orundesirable in a conductor/circuit. In a conductor,to transmit electricity from one place to anotherplace, the resistance is undesirable. Resistance in aconductor causes part of electrical energy to turninto heat, so some electrical energy is lost along thepath. On the other hand it is the resistance whichallows us to use electricity for light and heat e.g.,light that we receive from electric bulb and heat generated through electric heaters.

ACTIVITY 16.3

During your laboratory classes at your study centre, you can find the relation betweenthe current flowing through a wire and the potential difference applied across it withthe help of your tutor and your friends. Take a dry cell, a voltmeter (range 0-1.5V),an ammeter (range 0-1A), a standard fixed resistance coil (1 ohm), rheostat (0-1ohm), connecting wires and a plug key.

(i) Connect the fixed resistor (R), ammeter (A), dry cell (D), plug key (K) andrheostat (Rh) in series (end to end) and voltmeter (V) in parallel to R. as shownin Fig. 16.6 (a).

Fig. 16.6 (a) Circuit diagram to study relationship betweenvoltage and current

Resistor

0.25�

0.5�

1�

2�

Notes



370

(ii) When the key K is open, (meaning that the circuit is disconnected), check thatthe readings in ammeter and voltmeter are zero.

(iii) Insert the plug K in the key and move the sliding contact of the rheostate sothat there is some small reading in ammeter and voltmeter. Record thesereadings.

(iv) Increase the value of current with the help of rheostat. Record ammeter andvoltmeter readings again.

(v) After changing the readings 4 to 5 times, record the corresponding values ofcurrent and voltage from ammeter and voltmeter.

(vi) Plot a graph between ammeter and voltmeter readings.

What do you observe? You will observe that: (i) On increasing ammeter reading,voltmeter reading increases in the same proportion. (ii) The voltage-current graphis a straight line as shown in Fig. 16.6 (b).

Fig. 16.6 (b) variation of voltage with current

What do you conclude? We conclude that the current flowing through a wire isdirectly proportional to the potential difference applied across its ends.

i. e. V ∝ i

or V = Ri

Here, R is a constant of proportionality and is called the resistance of the given metallicwire. This observation was first made by Georg Simon Ohm and is known as Ohm’sLaw.

Ohm’s Law states that the current flowing through a conductor is directly proportionalto the potential difference applied across the ends of the conductor providedtemperature of the conductor remains the same.

Notes

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Now organize a brain storming session with your tutor and other learners on followingpoints. The law can be applied only to conducting wires and that too when itstemperature and other physical conditions remain unchanged. If the temperature ofthe conductor increases its resistance also increases.

‘R’ i.e. resistance of wire, is a constant for a given wire. It can be easily shown thatresistance of a wire depends on:

Its length - longer the wire, more the resistance

Its thickness - thicker the wire, lesser the resistance.

Its width – more the width, lesser the resistance.

Therefore, the resistance of the wire is directly proportional to the length and inverselyproportional to the cross-sectional area.

The nature of material - copper wire has lesser resistance than iron wire of samelength and thickness. The resistance of a wire can never be negative.

Resistance is a scalar quantity and its SI unit is ohm denoted by the symbol Ω(omega). 1 ohm is the resistance of a wire across which when 1V potential differenceis applied, 1A current flows through the wire.

i.e. 1 ohm =1 volt

1 ampere

High resistances are measured in kilo ohm (kΩ) and mega ohm (MΩ)

1 kΩ = 103 Ω

1MΩ = 106 Ω

16.4 COMBINATION OF RESISTORS

In an electric circuit, resistors can be connected in two different ways viz.

Series Combination: two or more resistors can be combined end to end consecutively.

Parallel Combination: two or more resistors can be connected between the sametwo points.

16.4.1 Series Combination

In a circuit (Fig. 16.7), three resistors are connected in series with a cell and anammeter. You will note that due to one path the same current i will flow through allof them.

Notes



372

Fig. 16.7 Resistors in series

Let the potential difference between the ends of the resistors R1, R2 and R3 arerespectively V1, V2 and V3

By ohm’s law potential difference across each resistor

V1 = iR1

V2 = iR2

and V3 = iR3

Now if the potential difference between P and Q be V

then V = V1 + V2 + V3

Substituting the values of the V1, V2 and V3

= iR1 + iR2 + iR3

= i(R1+ R2 + R3) (16.1)

Let total or equivalent resistance between P and Q is Rs

Then total potential difference V = iRs (16.2)

Comparing equations (16.1) and (16.2), we get

iRs = i(R1 + R2 + R3)

or Rs = R1 + R2 + R3

i.e. The equivalent resistance of three resistors connected in series is equal to thesum of their individual resistances.

16.4.2 Parallel Combination

Figure shows three resistors connected in parallel with a cell and an ammeter. Thepotential difference between points P and Q will be same across each resistor but

Notes

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the current flows from P to Q will be equal to the sum of the separate currents passingthrough each branch of a given resistance. If i1, i2 and i3 respectively represent thecurrent passing through the branches having the resistors R1, R2, and R3 then the totalcurrent i in the main circuit will be

Fig. 16.8 Resistors in parallel

i = i1 + i2 + i3 (16.3)

if V is the potential difference across each of the resistors, then according to Ohm’slaw

11

,V

iR

= 22

Vi

R= and 3

3

Vi

R= (16.4)

If RP is the equivalent resistance of the resistors connected in parallel having the samepotential difference V then

p

Vi

R= (16.5)

Using equations (16.4) and (16.5) the equation (16.3) will be

P

V

R =1 2 3

V V V

R R R+ +

i.e.1

PR =1 2 3

1 1 1

R R R+ +

i.e. the sum of the reciprocals of the separate resistances is equal to the reciprocalof equivalent or total or resultant resistor Rp.

Notes



374

Remember:

1. Normally all the appliances in our household circuits are connected in parallel.But the chain of small bulbs that we use for decoration on Deepawali has thebulbs connected in series.

2. As we add resistances in series, the circuit resistance increases but when weconnect resistances in parallel, the total resistance is smaller than the smallest ofthe resistances involved.

Multimeter is basically an AVO meter i.e., Ammeter, Voltmeter and Ohm meterwhich is used for measurement of current, voltage and resistance.

Example 16.3: A current of 0.5 A is drawn by a filament of an electric bulb for5th part of an hour. Find the amount of electric charge that flows through the circuit.

Solution: Given i = 0.5A1

5t = of an hour =

160

5× min = 12 min

Q = it = 12 × 60 s = 720 s

= (0.5A) × 720 s = 720 s

= 360 C

Notes

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Example 16.4: Find the equivalent resistance of the following combination ofresistors.

(a) (b)

(c)

Fig. 16.9

Solution:

(a) Here all resistors are connected in series.

R = 1 2 3 4 5 6 1 2 3 3 2 1 12 r r r r r r+ + + + + = + + + + + = Ω

(b) Here we have two series combinations of 3 resistors, each connected in parallel.

R1 = 1 2 3 6 + + = Ω

R2 = 1 2 3 6 + + = Ω

R =1 2

1 2

6 6 363

6 6 12

R R

R R

× ×= = = Ω+ +

(c) Here we have 3 parallel combinations of 2 resistors, each connected in series.

R =1 2

1 2

1 1 1

1 1 2

r r

r r

× ×= = Ω+ +

Notes



376

R =2 1

12 2

× = Ω+

R =3 3 9 3

1.53 3 6 2

× = = = Ω+

R = 1 2 31 3

1 32 2

R R R+ + = + + = Ω


1. Define the SI units of (i) current (ii) resistance.

2. Name the instruments used to measure (i) current (ii) potential difference.

3. Why is a conductor different from an insulator?

4. How is a volt related to an ohm and an ampere?

5. A number of bulbs are connected in a circuit. Decide whether the bulbs areconnected in series or in parallel, when (i) the whole circuit goes off when onebulb is fused (ii) only the bulb that get fused goes off.

6. When the potential difference across a wire is doubled, how will the followingquantities be affected (i) resistance of the wire (ii) current flowing through thewire?

7. What is the reading of ammeter in the circuit given below?

Fig. 16.10

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8. How can three resistors of resistance 2Ω, 3Ω and 6Ω be connected to give atotal resistance of (i) 11Ω (ii) 4.5Ω and (iii) 4Ω?

9. State two advantages of connecting electrical devices in parallel with the batteryinstead of connecting them in series.

16.5 HEATING EFFECT OF ELECTRIC CURRENT

It is a matter of common experience that on passing electric current through thefilament of an electric bulb, it gets heated and glows brightly. Similarly on passingcurrent through an electric heater, the coil of the heater becomes red hot. Do youknow why? It is because in an electric circuit, electrical energy is converted into heatenergy. This effect is known as thermal effect of electric current or Joules’ heating.

16.5.1 Heat produced in a conductor on passing electric current

Consider a conductor XY of resistance R. Let current ‘i’ is passed for t secondsthrough the conductor on applying a potential difference V across the ends X andY. If the charge Q is to be transferred from point X to Y, the work is done in movingthe charge Q across the ends of the conductor. Work done in transferring thecharge Q,

W = potential difference (V) × Charge (Q)

= Vit ( )Q it=∵

According to Ohm’s law V = iR

∴ W = (iR)it

W = i2Rt.

Here the work done in moving the electric charge across a resistance appears inthe form of heat. Therefore, the heat produced in the conductor is H = i2Rt.

Hence, the amount of heat produced in a conductor on passing the current i is directlyproportional to the square of the current (i2), the resistance of the conductor (R)and the time (t) for which the current flows through the conductor.

This is known as Joule’s law of heating. SI unit of heat is joule (J) (4.18 J = 1 cal)

16.5.2 Electric power

The rate at which electric energy is consumed or dissipated is termed as electricpower.

Electric power P =Work done ( )

Time taken ( )

W VitVi

t t= =

∴ P = Vi

Notes



378

or = (iR)i ( )V iR=∵

= i2R

or =2V

RR

⎛ ⎞⎜ ⎟⎝ ⎠

Vi

R⎛ ⎞=⎜ ⎟⎝ ⎠∵

=2V

R

SI unit of electric power is joule/second or watt (W) .Thus, from P = VI, unit ofpower is watt i.e 1 watt (W) = 1 volt (V) × 1 ampere (A).

Hence, electric power consumed in a circuit or a device is 1 W if a current of 1Aflows through it when a potential difference of one volt is maintained across it.

Since watt is a very small unit of power the bigger units kilowatt (kW) megawatt(MW) are actually used in practice.

1 kilowatt (kW) = 1000 W

1 megawatt (MW) = 106 W

1 gigawatt (GW) = 109 W

For electric power another bigger unit horse power (hp) is also used.

1 (hp) = 746 W

Since electrical energy consumed by an electrical appliance is equal to the productof power and the time for which it is used. The SI unit for the consumption of electricenergy is joule but it is very small from practical point of view. Therefore, the electricalenergy spent in the electric circuit is generally expressed in watt hour and kilowatthour.

1 watt hour is the amount of electric energy which is consumed in 1 hour in an electriccircuit when the electric power in the circuit is 1 watt.

1 kilowatt hour is the amount of electric energy consumed when 1 kilowatt poweris used for 1 hour in an electric circuit.

1 kilowatt hour (kW h) = 1 kilowatt × 1 hour

= 1000 watt × 3600 second

= 1000 joule/second × 3600 second

= 36 × 105 joule

1 kW h = 3.6 × 106 J

Notes

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To calculate the cost of electrical energy, special unit kilowatt hour (kW h) is usedwhich is also known as Board of Trade (BOT) unit or simply a unit of electricity.Therefore, the commercial unit of electric energy is kilowatt hour (kW h).

16.5.3 Electrical appliances based on thermal effect of electric current

There is a long list of household appliances based on thermal effect of electric currente.g electric iron, electric kettle, electric immersion rod/heater, electric geyser, cookingrange, electric oven, electric toaster, electric stove, room heater, etc.

Beside appliances heating effect of electric current is also used in electric fuse, electricwelding and electric arc. In all these appliances potential difference is applied acrossa conductor, the free electrons inside the conductor get accelerated and during thecourse of their motion electrons collide with other electrons and atoms/ions of thematerial of the conductor on their way and transfer their energy to them. The electronsmove with constant drift velocity and do not gain kinetic energy. But due to collisionwith free electrons, the atoms/ions begin to vibrate with increased amplitude .In otherwords, the average kinetic energy of vibrations of the atoms of conductor increaseswhich results in increase in temperature of the conductor i.e., the heat is producedin the conductor. Thus on applying potential difference, loss in potential energy ofthe electrons appears in the form of increase of average kinetic energy of the atomsof the conductor which finally appears as heat energy in the conductor

Electric Tester

It is used to indicate presence of electricity (a.c or d.c) in a circuit. It is like ascrew driver. This screwdriver has a handle, which can hold easily. It has a neonindicator bulb. The screw end of the tester is just touched with the chassis ofthe appliance like electric iron and a finger is kept on the clip of the tester toprovide earth. If the neon bulb glows upwith reddish light it shows that current ispassing through the chassis and it wouldgive a shock, therefore, it is essential toswitch off the mains immediately. If thelight does not glow, it indicates that thereis no leakage of current.

If you put a tester in an electric socketand if the neon bulb does not glow , itindicates that there is no power in theelectric socket. It is a must tool forelectrical automotive, electronic,appliance repairers.

Notes



380

ACTIVITY 16.4

You can do this simple activity with your friends to study thermal effect of electriccurrent. Take two pieces of the element of electric heater (one of which has 10 turnsand the other has 20 turns), two dry cells, connecting wires.

(i) Attach connecting wires to the free ends of the 10-turn coil permanently.

(ii) Touch the free ends of the connecting wires to the two terminals of a dry cell,thus passing current through it. Detach the contacts after 10 seconds. Now touchthe coil and feel it.

(iii) Repeat the experiment by passing current for 20 seconds.

(iv) Place two dry cells in contact, making series battery and repeat the second step.

(v) Repeat steps 2, 3, 4 with 20-turn heater coil and feel it.

(a) (b) (c)

Fig. 16.11 Study of thermal effect of electric current

Discuss the observations with your friend, you will observe that on passing currentthrough a conductor it gets heated up. The coil is found to be heated when currentis passed for a second. The coil is found to be hotter when greater voltage is appliedacross it. When same voltage is applied across bigger coil less heat is produced init. Thus, we conclude that

(i) Current has a heating effect, i.e. when current is passed through a conductorit gets heated up.

(ii) More heat is produced in a conductor

when more potential difference is applied across it.

current is passed through it for more time (t).

more current is passed through the same conductor.


1. Which will produce more heat in 1 second – 1 ohm resistance on 10V or a 10ohm resistance on the same voltage? Give reason for your answer.

Notes

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2. How will the heat produced in a conductor change in each of the following cases?

(i) The current flowing through the conductor is doubled.

(ii) Voltage across the conductor is doubled.

(iii) Time for which current passed is doubled.

3. 1 A current flows though a conductor of resistance 10 ohms for 1/2 minute. Howmuch heat is produced in the conductor?

4. Two electric bulbs of 40 W and 60 W are given. Which one of the bulbs willglow brighter if they are connected to the mains in (i) series and (ii) parallel?

5. How is 1 kW h related with SI unit of energy?

6. Name two household electric devices based on thermal effect of electric current.

There are three types of large scale electric power generating plants

(i) Hydroelectric power plants – when potential energy of water stored in a damis used for generating electricity. e.g. Bhakra- Nangal hydroelectric powerplant, Punjab.

(ii) Thermal power plant – where a fossil fuel is burnt to produce steam whichruns a turbine to convert mechanical energy into electrical energy. e.g.Namrup thermal power station, Assam.

(iii) Atomic power plant – where nuclear energy is obtained from a fissionablematerial like uranium is used to run a turbine. e.g. Narora atomic powerstation, Uttar Pradesh.

In India all the major plants produce A.C. (alternating current) at 50 hertz, 11000volts or more. This power can be further stepped up to higher voltages usingtransformers and hence can be transmitted to long distances without much lossof power.

1. Alternating current (AC) means the electric current is alternating directionsin a repetitive pattern.

2. AC is created by generators in power plants, and other sources. This ACcurrent is delivered to our homes and businesses by the power lines we seeeverywhere.

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382

Example 16.5: Find the resistance of the filament of 100 W, 250 V electric bulb.

Solution: R =2V

P

=250 250

625100

× = Ω

Example 16.6: Calculate the energy consumed in a 2 kW electric heater in 2 hours.Express the result in joules.

Solution: Q = Pt = 2 kW × 2 h = 4 kW h

= 4 × 3.6 × 106J = 14.4 × 106 J

Example 16.7: How much time will take a 2 kW immersion rod to raise thetemperature of 1 litre of water from 30°C to 60°C

Solution: Q = Pt

Q = mcθ

mcθ = Pt (1)

Mass of 1 litre of water (m) = 1 kg

Specific heat of water c = 4.18 × 103 J kg–1 °C–1

Rise in temperature of water (θ) = 60 – 30 = 30°C.

P = 2 kW = 2000W

Substituting in equation (1) we get

1 × 4.18 × 103 × 30 = 2000 × t

t =3

3

125.4 1062.7s

2 10

× =×

Example 16.8: How many kilowatt hour of energy will be consumed by a 2 hpmotor in 10 hours?

Solution: P = 2 hp = 2 × 746 W

= 1.492 kW

Q = Pt = 1.492 kW × 10 h = 14.92 kW h

Notes

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Example 16.9: A potential difference of 250V is applied across a resistance of 1000ohm. Calculate the heat energy produced in the resistance in 10 s.

Solution: Given V = 250 V R = 1000 W t = 10 s

Q =2 250 250 10

625J1000

V t

R

× ×= =

Example 16.10: Compute the heat generated while transferring 96 kC of chargein one hour through a potential difference of 50V.

Solution: Given: V = 50V t = 1 h q = 96000 C

W = qV

= 96000 C × 50 V

W = 4800000 J

= 4.8 × 106 J

= 4.8 MJ.

Example 16.11: An electric iron of resistance 25 Ω takes a current of 5A. Calculatethe heat developed in 1 minute.

Solution: Given: R = 25 Ω i = 5A t = 1 min (= 60 s )

Heat developed H = i2Rt

= (5 A)2 ×25 Ω × 60 s

= 37500 J = 3.75 ×104 J


1. Which has a higher resistance, a 40W-220 V bulb, or a 1 kW-220V electricheater?

2. What is the maximum current that a 100W, 220 V lamp can withstand?

3. How many units of electricity will be consumed by a 60 W lamp in 30 days ifthe bulb is lighted 4 hours daily?

4. How many joules of electrical energy will a quarter horse power motor consumein one hour?

5. An electric heater is used on 220 V supply and draws a current of 5 A. Whatis its electric power?

6. Which uses more energy, a television of 250 W in 60 minutes or a toaster of1.2 kW in (1/6)th of an hour?

Notes



384

Symbols used in Electric Circuit Diagram

Notes

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� The force of attraction between the electrons and the protons hold an atomtogether.

� When two bodies are rubbed together in contact, they acquire a peculiar propertyof attracting small bits of paper. We say the bodies are electrified or chargedby friction.

� Charges are of two types. Charge acquired by a glass rod rubbed with silk ispositive and that acquired by an ebonite rod rubbed with fur is negative.

� Like charges repel each other and unlike charges attract each other.

� The force between two charges is given by Coulomb’s law according to which

1 22

kq qF

r= . The closer together the charges are, the stronger is the electrostatic

force between them.

� Potential is the electrical state of a conductor which determines the direction offlow of charge when the two conductors are either placed in contact or they areconnected by a metallic wire.

� Work is done in moving a charge against electric field which is stored up aspotential energy of the charge. Hence, when charge is placed at a point in thefield it possesses potential energy.

� Potential energy per coulomb of charge at a point is called potential. Positivecharge always moves from a higher potential to a lower potential and vice-versa.

� The potential at a point is the amount of work done in bringing a unit positivecharge from infinity to that point.

� The potential difference between two points is the amount of work done in movinga unit positive charge from one point to the other.

� Electric current at a place is the charge passing per unit time through that place.

� Electric cell is a device with the help of which we can apply a potential differencebetween the two ends of a wire due to which current will flow through the wire.

� Circuit diagrams are used to show how all the components connect together tomake a circuit.

� Ohm’s law states that current flowing through a conductor is directly proportionalto the potential difference applied across its ends, provided physical conditionstemperature etc.of the conductor remain the same.

� The obstruction offered to the flow of current by the wire is called its resistance.Mathematically ratio of voltage applied across a conductor and the current

Notes



386

flowing through it is called resistance of the conductor. SI unit of resistance isohm.

� Resistors may be connected in two different independent ways

(i) In series and (ii) in parallel.

� In series, total resistance of the combination is equal to the sum of the individualresistances.

� In parallel, reciprocal of the combined resistance is equal to the sum of thereciprocals of the individual resistances.

� When current is passed through a conductor, it produces two effects.

(i) Thermal effect (ii) Magnetic effect.

� Commercial unit of electrical energy is kW h and that of electric power is HP.

For more information:

1. Multimedia CD on Innovative physics experiments developed by Vigyan Prasar,Department of Science & Technology,Govt of India. www.vigyanprasar.gov.in

2. Multimedia CD on Fun with Physics developed by Vigyan Prasar, Departmentof Science & Technology,Govt of India. www.vigyanprasar.gov.in

3. Flying circus of Physics by Jearl Walker, John Wiley and sons Publication.

TERMINAL EXERCISE

1. Tick mark the most appropriate answer out of four given options at the end ofeach of the following statements:

(a) A charged conductor ‘A’ having charge Q is touched to an identicaluncharged conductor ‘B’ and removed. Charge left on A after separationwill be:

(i) Q (ii) Q/2 (iii) Zero (iv) 2Q

(b) J C–1 is the unit of

(i) Current (ii) Charge (iii) Resistance (iv) Potential

(c) Which of the following materials is an electrical insulator?

(i) Mica (ii) Copper (iii) Tungsten (iv) Iron

(d) The device which converts chemical energy into electrical energy is called

(i) Electric fan (ii) Electric generator

(iii) Electric cell (iv) Electric heater

Notes

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(e) The resistance of a conductor does not depend on its

(i) Temperature (ii) Length (iii) Thickness (iv) Shape

(f) There are four resistors of 12 Ω each. Which of the following values ispossible by their combination (series and/or parallel)?

(i) 9 Ω (ii) 16 Ω (iii) 12 Ω (iv) 30 Ω

(g) In case of the circuit shown below in Fig. 16.12, which of the followingstatements is/are true:

(i) R1, R2, and R3 are in series

(ii) R2 and R3 are in series

(iii) R2 and R3 are in parallel

(iv) The equivalent resistance of the circuit is [R1+ (R2 R3/R2 + R3)]

Fig. 16.12

(h) The equivalent resistance of two resistors of equal resistances connectedin parallel is —— the value of each resistor.

(i) Half (ii) Twice (iii) Same (iv) One fourth

2. Fill in the blanks.

(a) When current is passed through a conductor, its temperature .................

(b) The amount of ................ flowing past a point per unit ................ isdefined as electric current.

(c) A current carrying conductor carries an ................ field around it.

(d) One ampere equals one ................ per .................

(e) Unit of electric power is .................

(f) Of the two wires made of the same material and having same thickness,the longer one has ................ resistance.

3. How many types of electric charge exist?

4. In a nucleus there are several protons, all of which have positive charge. Whydoes the electrostatic repulsion fail to push the nucleus apart?

Notes



388

5. What does it mean to say that charge is conserved?

6. A point charge of +3.0 μC is 10 cm apart from a second point charge of–1.5 μC. Find the magnitude and direction of force on each charge.

7. Name the quantity measured by the unit (a) VC (b) Cs–1

8. Give a one word name for the unit (a) JC–1 (b) Cs–1

9. What is the potential difference between the terminals of a battery if 250 J ofwork is required to transfer 20 C of charge from one terminal of the batteryto the other?

10. Give the symbols of (a) cell (b) battery (c) resistor (d) voltmeter.

11. What is the conventional direction of flow of electric current? Do the chargecarriers in the conductor flow in the same direction? Explain.

12. Out of ammeter and voltmeter which is connected in series and which isconnected in parallel in an electric circuit?

13. You are given two resistors of 3 Ω and 6 Ω, respectively. Combining these tworesistors what other resistances can you obtain?

14. What is the current in SI unit if+100 coulombs of charge flows past a pointevery five seconds?

15. Deduce an expression for the electrical energy spent in flow of current througha conductor.

16. Find the value of resistor X as shown in Fig. 16.13.

Fig. 16.13

17. In the circuit shown in Fig. 16.14, find (i) Total resistance of the circuit.(ii) Ammeter reading and (iii) Current flowing through 3 Ω resistor.

Fig. 16.14

Notes

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18. For the circuit shown in Fig. 16.15, find the value of:

(i) Current through 12Ω resistor.

(ii) Potential difference across 6Ω and 18Ω resistor.

Fig. 16.15

19. You are given three resistors of 1 Ω, 2 Ω and 3 Ω. Show by diagrams, howwill you connect these resistors to get (a) 6/11 Ω (b) 6 Ω (c) 1.5 Ω?

20. A resistor of 8 Ω is connected in parallel with another resistor of X Ω. Theresultant resistance of the combination is 4.8 ohm. What is the value of resistorX?

21. In the circuit Fig. 16.16, find

(i) Total resistance of the circuit.

(ii) Total current flowing through the circuit.

(iii) The potential difference across 4 Ω resistor.

Fig. 16.16

22. How many 132 Ω resistors should be connected in parallel to carry 5 A currentin 220 V line?

Notes



390


16.1

1. (i) Unit of charge is Coulomb. 1C charge is the charge which when placed ata distance of 1 m from an equal like charge repels it with a of force of9 × 109 N.

(ii) Unit of potential is volt. 1 volt is the potential at a point in an electric fieldsuch that if 1C positive charge is brought from outside the field to this pointagainst the field 1 J work is done.

2.6

1319

10 106.25 10

1.6 10

QN

e

−

−×= = = ××

electrons

3. 1 2 1 22 2

2 28F

( / 2)

q q q qF k F k

r r

×= ⇒ = =

4. F′=1/4 F

5. (i) 76

1010 V

10

UV

q −= = =

(ii) 39 6

10 0.5 510 C

99 10 10

KQq UrU Q

r Kq−

−×=

× ×

6. Electrons will flow from sphere B to sphere A through the wire till the potentialsof the two spheres become equal.

16.2

1. (i) Unit of current is ampere. 1A is the current in a wire in which 1C chargeflows in 1 second.

(ii) Unit of resistance is ohm. 1 ohm is the resistance of a wire across whichwhen 1V potential difference is applied, 1A current flows through it.

2. (i) ammeter (ii) voltmeter

3. A conductor has free electrons, whereas an insulator has no free electrons.

4. 1 volt = 1 ohm × 1 ampere

Notes

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5. (i) If the whole circuit goes off when one bulb is fused, the bulbs are connectedin series.

(ii) If any one bulb goes off and the rest of the circuit remains working, the bulbsare connected in parallel.

6. (i) Resistance of the wire remains unaffected.

(ii) Current flowing through the wire is doubled.

7. 1A

8. (i) All the three resistors are connected in series.

(ii) Resistors 2Ω and 6Ω are connected in parallel and 3Ω is connected in seriesto the combination of 2Ω and 6Ω.

(iii) Resistors 3Ω and 6Ω are connected in parallel and 2Ω is connected in seriesto the combination of 3Ω and 6Ω.

9. In a parallel circuit, every electrical gadget operates separately because they takecurrent as per their requirement.

Total resistance of the circuit is decreased.

If one component fails, the circuit is not broken and other electrical devices workproperly.

16.3

1. Q/t = V2/R. This implies that more the resistance less the power. Therefore, moreheat will flow in 1s in 1 ohm resistor.

2. (i) Heat produced becomes four times (ii) heat produced becomes four times(iii)heat produced will be doubled.

3. Q = i2Rt = 1 × 10 × 30 = 300 J.

4. P = V2/R and energy consumed in series = i2Rt and in parallel = (V2/R)t

(i) The bulb with lowest wattage (highest resistance) glows with maximumbrightness.

(ii) The bulb with highest wattage (lowest resistance) glows with maximumbrightness.

5. 1 kW h = 3.6 × 106 J

6. (i) Electric heater (ii) Electric kettle

Notes



392

16.4

1.2V

RP

= , 40W lamp has higher resistance.

2.100 W 5

A.220 V 11

PI

V= = =

3. Q = Pt = 60W × 4h × 30 = 7200 W h = 7.2 kW h

4. 746

W×3600s 671400 J.4

Q Pt= = =

5. P = VI = 220 V × 5A = 1100 W

6. Energy used by television = 0.25 kW × 1 h = 0.25 kW h

Energy used by toaster = 1.2 kW × 1/6 h = 0.2 kW h

Notes

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Magnetic Effect of Electric Current


17

MAGNETIC EFFECT OFELECTRIC CURRENT

In the earlier lesson you have learnt that, electricity is an important part in today’sworld of industrialization. Our life is incomplete without it. Whether we work in anoffice or at home, every thing depends upon the availability of electricity. Applianceslike the electric bulb, fan, television, refrigerator, washing machine, motor, radio,everything works due to electricity.

When electric current passes through current carrying conductor or coil then amagnetic field is produced around it. The working of appliances like electric bell isbased on this principle. As opposite to this if a continuous change in magnetic fieldis produced then electric current can be produced. This is how electricity andmagnetism have become synonymous today. Transmission of electric current takesplace from the distant electricity generation stations through high tension wires,through transformers to homes. This chapter deals with the meaning of safe use ofelectricity. Along with this the same concepts related to magnetism are explainedthrough simple activities that one can perform on their own.

OBJECTIVES

After studying this lesson you will be able to:

� identify magnets and explain their properties;

� explain the concept of magnetic field and state the properties of lines ofmagnetic force;

� infer that when electricity flows through a conductor, magnetic field isproduced around it;

� describe electro-magnets and explain the working of electric bells;

� explain the force experienced by a current carrying conductor placed in amagnetic field;

Notes


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394

� describe electromagnetic induction and its importance in different aspectsof daily life;

� describe alternate current (AC) and direct current (DC) currents and listthe appliances that work on these currents;

� state the hazards involved in using electrical energy in industries and athome and describe safety measures necessary to minimize them.

17.1 MAGNET AND ITS PROPERTIES

Magnet has always been a thing of awe use and attraction for humans. Accordingto history, the use of magnets were discovered by the ancient Greeks during theperiod of Greek Civilization.

Fig. 17.1 Natural magnet

They found stones which were able to attract iron and nickel like other substances.This naturally occurring stones (see Fig. 17.1) which was discovered then is calledas ‘lodestone’. This is an oxide of iron (Fe3O4). The property of attraction of smallparticles of iron towards lodestone is called as ‘magnetism’. It has been often seenthat the magnetic force of attraction of these natural magnets is much less and thus,these magnets cannot be use for practical purposes. Strong magnets made of iron,nickel and lead are made artificially and used for practical purposes. Those magnetsare also called as permannent magnet. So, a magnet is a material or object thatproduces a magnetic field which is responsible for a force that pulls or attacts onother materials.

These strong magnets can be made in various shapes and creats its own persistentmagnetic field. The magnets that are commonly available in different shapes are:

(a) Bar magnet (b) Horseshoe magnet

(c) Cylindrical magnet (d) Circular magnet

(e) Rectangular magnet

Notes

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Fig. 17.2 Magnets of different shapes

Have you ever observe magnets of any of above shapes arround you? These magnetsof different shapes are used in various appliances used at home like tape recorder,radio, motor, door-bell, head phones etc. These magnets are used in variousappliances to either hold or separate, control, elevate (lift) substances, changingelectrical energy to mechanical energy (motors, loudspeakers) or mechanical toelectrical energy (generators and microphones).

If a natural magnet is suspended freely with the help of a string, it always rests inthe ‘north-south’ direction. If the magnet is slightly turned from this direction, it stillreturns to the same. The end that rests towards the ‘north’ is named as ‘North Pole’while the one which ends at ‘south’ is named as South Pole. They are representedas ‘N’ and ‘S’.

ACTIVITY 17.1

Take one magnetic needle, two bar magnets, some iron filling, an alpin and do theexperimental study of properties of magnet.

Following step may followed:

1. Tie a string at the middle of a bar magnet and hang it with the help of a hook.This bar always rests at the same direction. With the help of the magnetic needle,

CircularMagnet

CylindricalMagnet

RectangularMagnet

Horse ShoeMagnet

Bar Magnet

Notes



396

find out the direction. By this you will be able to prove that a bar magnet alwaysrests in the north-south direction.

Fig. 17.3 (i)

2. Take iron fillings near the bar magnet. They stick to the magnet. Thus, magnetattracts iron. You would observe that the amount of iron fillings near the polesis maximum while at the middle is negligible.

Fig. 17.3 (ii)

3. If you bring any pole of a bar magnet near the pole of a freely suspended barmagnet, then either it will attract or repel the same. Opposite poles of a magnetattract each other while like poles (north-north or south-south) repel each other.

Fig. 17.3 (iii)

Notes

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4. Take an iron alpin near a bar magnet leave it there for sometime. You will findthat the alpin has acquired magnetic properties and iron fillings start sticking tothe ends of the alpin as well.

Fig. 17.3 (iv)

5. Break the bar magnet into smaller pieces. Now observe that magnetic propertiesare retained in the pieces as well. Thus, the two poles of a magnet cannot beseparated.

Fig. 17.3 (v)

17.1.1 Properties of Magnets

Through the activity 17.1 we can list the properties of magnetic as follows:

1. Attracts iron towards itself.

2. Freely suspended magnet always rests at the north-south direction.

3. Like poles repel while unlike poles attract.

4. If iron pieces are brought near a strong magnet they also start behaving asmagnets.

5. The poles of a magnet cannot be separated.

17.2 MAGNETIC FIELD

Keep a small magnetic needle near a bar magnet. The magnetic needle rotates andstops in a particular direction only. This shows that a force acts on the magnetic needlethat makes it rotate and rest in a particular direction only. This force is called torque.

The region around the magnet where the force on the magnetic needle occurs andthe needle stops at a specific direction, is called a magnetic field. The directionof magnetic field is represented by magnetic line of forces. As shown in Fig. 17.4(i),

Notes



398

the direction of magnetic needle changes continuously and it takes the curved pathwhile moving from north to south. This curved path is known as magnetic line offorces. Tangent line draw at any point on magnetic line of force, represent the directionof magnetic field at that point. These magnetic line of forces have following properties:

1. Magnetic line of forces always start from north pole and end at south pole ofthe magnet.

2. These line of forces never intersect each other.

3. Near the poles magnetic lines are very near to each other which shows thatmagnetic field at the poles is stronger as compare to other parts.

Fig. 17.4 (i)

Our Earth itslef acts as a giant magnet with south magnetic pole somewhere in theArctic and north magnetic pole in Antaractic. The Earth alsobehaves like a bar magnet. It’s hot liquid centre core containsiron and as it moves, it creates an electric current that causea magnetic field around the Earth. The Earth has a north andsouth magnetic pole. These poles are not same with thegeographic north and south poles on a map and tilted at anangle of 11.3 degree with repect to it. Due to this, if a magneticneedle is suspended freely, it rests in the north-south directionand is useful for navigation.

ACTIVITY 17.2

You can also detect the presence of magnetic field. For doing this take two barmagnets, one scale and follow the given steps:

Fig. 17.4 (ii)

Notes

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1. Keep two bar magnets in such a way that they are laid on the same line andin same plane at a distance of 10 cm.

2. Bring the like poles slowly towards each other. Could you feel something?

3. You will feel two poles are repelling each other.

4. Change the orientation of one of the bar magnets so that opposite poles faceeach other. You would observe that the two magnets quickly come close togetherdue to force of attraction between them.

With this we can conclude that force acts between the magnets and the region aroundthe magnets in which this force may be experienced is called the ‘magnetic field’.

Fig. 17.5

INTEXT QUESITONS 17.1

1. Define magnet and list its properties?

2. What happen, with the properties of magnet when it is broken into two pieces?

3. Name the part of telephone where magnet is used?

4. Hang the bar magnet with the string, it will always rest in which directions

(i) East-west (ii) West-south

(iii) North-south (iv) North-east

5. Do magnetic field exist throughout space?

6. The north pole, magnetic needle points towards earth’s

(i) North pole (ii) South pole

(iii) Centre (iv) None of the above

7. What are magnetic poles?

Notes



400

17.3 MAGNETIC FIELD AROUND THE CURRENTCARRYING WIRE

If an electric current is made to flow in a wire, magnetic field produce arround it.For seeing this take a conducting wire (like copper). Now with the help of connectingwires attach this to the two ends of a battery. Keep a magnetic needle parallel tothe conducting copper wire as shown in Fig. 17.6(a). When the circuit is completethe magnetic needle shows deflection. This shows that when electric current flowsthrough a conductor, magnetic field is produced around the conductor. If the currentis increased, there is greater amount of deflection. If the direction of flow of electriccurrent is changed (by reversing the end of the battery) the direction of deflectionin the magnetic needle is also reversed. If the current flow is stopped the deflectionin the magnetic needle also ceases. Thus magnetic field is an effect of flow of electriccurrent through conducting wire. In the year 1820 a scientist from Denmark namedH.C. Oersted observed this effect for the first time.

(a) (b)

Fig. 17.6

The principle of the magnetic effect of electric current used in many appliances likemotor etc.

17.4 ELECTROMAGNET

An electromagnet is a type of magnet in which magnetic field is produced by theflow of elctric current. For making electromagnet take a piece of paper and givea cylindrical shape.

Make several turns of a copper wire over this from one end to the other. This iscalled solenoid a long thin loop of wire. When the ends of the copper wires areattached to the ends of a battery (+ and –) current starts flowing through the coilwhich starts functioning as a bar magnet.

H.C. Oersted (1770-1851)

Notes

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When the flow of current is stopped from the battery, then, its magnetic propertyceases. If the +ve and –ve terminals of the battery are reversed, then the poles ofthe magnet are also reversed.

Fig. 17.7 Solenoid

For increasing the magnetic field, put soft iron like iron nails inside the core. SoCurrent carry solenoid with soft iron core inside it forms an electromagnet.Electromagnet may be made as strong as one may desire. Electromagnet are widelyused as a components in electrical devices such as motor, generator, electrical bellsMRI machine etc. Beside that strong electromagnet are also being used in breaksystem of the superfast train in the world, in the cyclotrons and in mega experimentslike experiment at CERN laboratory at Geneva. The comparison of a bar magnetand an electromagnet has been illustrated in the table given below.

17.4.1 Difference between a Bar Magnet and an Electromagnet

Bar magnet Electromagnet

This is a permanent magnet. Its magnetic This is a temporary magnet. Its magnetismfield remains constant. remains till current flows through it.

Its magnetic strength cannot be reduced Its magnetic strength may be changed at willor increased. by changing the amount of current flow.

This is a weak magnet. This is a strong magnet. Strength of magneticfield can be controlled.

The poles do not change. By mere change in the direction of flow of

electric current the poles may be altered.

ACTIVITY 17.3

Let us try to make an electromagnet with our hands. For this take thick paper likedrawing sheet, copper wire, 9V battery or eliminator through which mill amperecurrent may flow, switch and iron scale and follow the given steps.

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1. Make a cylindrical tube of 15 cm. long with a diameter of 1 cm. by rolling thethick paper sheet.

2. Make around 100 to 150 coils of copper wire around this tube. Please notethat core is empty here.

Fig. 17.8 (a)

Fig. 17.8 (b)

(i) (ii)

(iii)

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3. Connect the end of the wires with the help of a switch to the ends of the battery.

4. Take an iron scale near the tube before the switch is on.

5. You will see that no force may be felt over the iron scale.

6. Now the switch it on and allow the current to flow.

7. As current flows the iron scale is pulled towards the tube. This shows that thecylindrical tube works as a magnet. This magnetic property occurs due tosolenoid.

8. Now fill iron nails inside the tube (core). You will observe that there is a greaterforce pulling at the scale. This shows that the electromagnet has become stronger.This happened because from atoms inside the core line up and increase magneticfield.

9. As the current flow is stopped, the magnetic effect of the tube is also lost.

17.4.2 Electric Bell

How does electrical bell work? This electrical device where electromagnetic is usedas components. In electric bell ‘U’ shaped electromagnet is used. This is also calledhorse shoe electromagnet. Soft iron is placed between the arms of this electromagnet.This is called as ‘core’.

Fig. 17.9 Electric bell

The poles of the electromagnet are connected to a power supply (battery or main).Between this a push button (B) is attached as shown in Fig. 17.9. When the pushbutton is pressed, current starts flowing in the coil of the electromagnet and its softiron ‘core’ gets magnetized. This magnetized core pulls the armature attached to the

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electromagnet towards itself. As a result of this, the hammer attached to the armaturestrikes the gong and a loud sound is produced. As soon as the armature is attractedby electromagnet the circuit is brought at the contact screw. The electromagnet nomore remains magnetic. The armature returns to its original position due to armaturespring action.

This process occurs repeatedly. Till the push button remains pressed, the hammerkeeps striking the gong of the bell and as a result of this, sound is produced.

17.5 FORCE ON A CURRENT CARRYING CONDUCTORPLACED IN A MAGNETIC FIELD

You have seen earlier that, when current flows through a conductor, magnetic fieldis produced around it. The direction of this field (B) depends upon the direction offlow of electric current (I). Similarly when an electrical conductor is placed in amagnetic field a force acts upon it. The following experiment may be done to observethis.

Let us suspend a piece of copper wire between the poles of a horse shoe magnetin such a manner that the length of the wire is aligned perpendicular to the directionof magnetic field between the poles. As soon as current is allowed to flow throughthis wire it becomes taut upwards. With this becomes clear that a force acts on thecurrent flowing conductor. The direction of this force always perpendicular to bothdirection of current and direction of magnetic field and the direction of the flow ofcurrent are both perpendicular to the direction of the magnetic field. If the magnetis flipped i.e. the poles are reversed, the conducting wire becomes taut downwards.This force acts on the wire downwards. If the current flowing through the conductoris increased then the force also increases. This force acting on a current conductingwire was discovered by the great scientist Michael Faraday. This principle is usedin electric motors.

(a) (b) Force on a current carrying conductor

Fig. 17.10

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The direction of force acting on a current carrying conductor placed in a magneticfield can be found according to the following rule:

Flemings left hand rule

According to Fleming left hand rule the direction of force applied to a current carryingwire is perpendicular to both the direction of the current as well as the magnetic field.It means that, stretch the thumb, the first finger andthe middle finger in such a manner that they areperpendicular to each other i.e. the angle betweenthe pairs of fingers is 90°. Then if the first fingershows the direction of the magnetic field and themiddle one the direction of current flow, then thethumb shows the direction of force ‘F’ acting onthe current carrying conductor. This rule wasoriginated by John Ambrose Fleming in the late19th century as a simple way of workout thedirection in an electric motor or the direction ofelectric current in an electric generator.

INTEXT QUESITONS 17.2

1. Presence of magnetic field around an electric wire can be proved by which anbehaviour of iron filings

(i) They form a circular patterns as soon as the electricity is turned on.

(ii) Try filing fly off the card when the current is on

(iii) They do not prove anything, because it is magic trick.

(iv) None of the above

2. Among these which property is not belong to electromagnet?

(i) It is permanent magnet

(ii) Its magnetic strength can not be decreased or increased at as well

(iii) Its polarity can be remove by reversing flow of electric current.

(iv) It produce strong magnetic field.

3. To findout the direction of force in the electric motor which rules is used?

(i) Flemings right hand rule

(ii) Flemings left hand rule

Fig. 17.11 Flemings left hand rule

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(iii) Right plam rule of right hand

(iv) Left palm rule of left hand

4. Why does an iron core increase the magnetic field of a coil of wire

(i) The iron atom line up to add the magnetic field

(ii) Iron attract things includes magnetic fields

(iii) The iron core actually decrease the field, allowing it to be turned off

5. List the factors affecting the strength of an electromagnet?

6. What is the role of solenoids in making electromagnet?

17.6 ELECTROMAGNETIC INDUCTION

Previously in this lesson, we have seen that magnetic field is created when currentflows through a solenoid (a cylindrical core of insulated copper wires). Do you thinkthat the reverse should also be possible? That means conversion of electricity frommagnetism. Michael Faraday, a great scientist also think over it and gave a discoveryof induction in 1831. After several years of continuous experimentation he discoveredthat if changes are brought about in the magnetic field then electric current can beproduced. If we rotate a coil of a good conducting wire between the poles of amagnet, then the number of magnetic lines of force associated with it are altered.Similarly if a magnet moves within the coil there is change in the same manner. Whenthis occurs, current starts flowing in the coil. So electromagnetic induction is theproduction of an electric current across a conductor moving through a magnetic field.Generators, transformers some devices which works on this principle.etc workingon this principle.

ACTIVITY 17.4

With this activity, we would able to see, how electricity generated through a magneticfield. You will required a strong magnet, copper wire, pipe (a non conductor), acurrent measuring instrument like galvanometer, a non conductor pipe (for examplemade up of cardboard, bamboo etc.) on which copper wire is wound to form a coils.First connect both ends of copper wire to the galvanometer, (Fig. 17.12a) keep themagnet parallel to the coil, bring it close to it and take it away. Repeat the processseveral times. You will see that each time there is a deflection in the galvanometer.With this you will be also observe that the see that the rate of flow of electric currentthrough the coil increases with the rate of change of magnetic field, the faster thechange the greater is the amount of a current flow.

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(a)

(b)

Fig. 17.12

To understand the above principle better, we will try to make a simple experiment,Take a disposable syringe (the one with which a doctor administers injections). Makea 150 turns thin copper insulating wire at the middle of the syringe. Join the two endsof copper wire to Light Emitting Diode (LED) through connecting wire. LED maybe fixed inside the plastic bottle capas in Fig. 17.12(b). Placed one cylindrical magnetinside the syringes. When you move magnet, holding the syringe in your hand,continues change in magnetic field take place and LED start glowing.

17.7 ELECTRIC GENERATOR

Electric Generator is such a device that converts mechanical energy to electricalenergy. Generators are of two types:

1. A.C. Generator (Alternating Current Generator): This produces current thatflows in such a manner that its direction and amplitude changes constantly withtime.

2. D.C. Generator (Direct Current Generator): This generator produces currentthat flows in the same direction in a continuous manner.

17.7.1 Structure and Function of an A.C. Generator

A.C. generators operate on the principle of electromagnetic induction. Alternatingvoltage or current may be generated by rotating a coil in the magnetic field or by

G

LED bulb

Syringe

Copper wire

Magnet

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rotating a magnetic field with a stationary coil. The value of the voltage or currentgenerated depends on

� the number of turns in the coil

� strength of the field

� the speed at which the coil or magnetic field roates

The structure of an A.C. Generator has been shown in Fig. 17.13. Here N-S is astrong permanent magnet. ABCD is a nonconductor frame on which copper wirehas been coiled several times to form a rectangular coil. The coil is coated with anonconductor substance like varnish so that they do not touch each other. This coilcan freely move between the N-S poles. This rectangular coil is made to rotatebetween two rings E and F. There are two contact brushes G and H attached tothe rings respectively.

Fig. 17.13 A.C. Generator

The rectangular frame ABCD moves between the N-S poles due to mechanicalenergy. Assume that the plane of the coil in that of the magnet lines of force andthe coil starts moving in an anti-clockwise direction. The magnetic field entering intothe face ABCD of coil increase from zero to some infinite value and continues toincrease till the coil becomes normal to the field. The rate, at which the magneticfield liked with coil charges, is the maximum in the begining and then it decreasecontinuously. Thus the induced current in the coil is maximum at time, t = 0 anddecrease passing time. When the coil become normal to the field the rate at whichmagnetic flux of force charges become zero and hence current in the coil is zero.

When the coil further rotates the face of the coil through which magnetic field entersstart changing the directing of current reversed. It keeps increasing till the plane ofthe coil does not become parallel to the magnetic field lines. Thus maximum currentflows through the coil at this juncture. If the coil is rotated further, the area in contact

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with DCBA increase and the rate of change of magnetic field area becomes less.Thus the amount of current flowing through the coil decreases. When the coil isperpendicular to the magnetic lines of force then, current becomes zero. Now thenorth pole of the magnet is reversed. Current starts flowing from its original direction.The direction of current produced and its resultant keeps changing with time. Figure17.14 shows the positions of the coils at different stages and the current in the coilat these instants.

Fig. 17.14

17.7.2 D.C. Generator

This also works like the AC generator. There is just one difference in its structure.There is a half rectangular rings rather than E and F rings as seen in AC generators.The rectangular frame rotates and moves from a position parallel to the magneticfield to an upper one. The brush present creates a connection as electric current startsflowing. We can see thus that current flows in the same direction.

Fig. 17.15 D.C. Generator

17.7.3 Alternative Current AC and Direct Current (DC)

In household as well as industrial purposes AC is widely used. The current that flowsout from the switch points at homes is AC. The current produced by a battery is

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DC. AC can be changed to DC and vice versa. To change AC to DC a rectifieris used.

1. AC is transmitted from electricity generation centers to houses and industriesthrough high voltage transformer (step up transformers) at very high voltage. Atthe site of delivery like houses and industries the voltage is reduced with the helpof step down transformers. In this way transmission cost is reduced as well aswastage of energy is minimized. Transmission of DC causes the loss of a largeamount of energy. Transformers cannot be used for DC.

2. Devices like electric motor that work on AC are stronger than those that useDC. They are also more convenient to use. DC generally used in electrolysis,changing the cells, making electromagnet etc.

3. DC of same voltage as AC is more dangerous because in DC direction of flowof current doesn’t change. Thus people coming in contact with DC accidentallyget stuck to it while when the come in contact with AC, due to change in directionof flow of current they are flung afar.

4. Major portion of AC flows through the upper portion of a wire. Thus where athick wire has to be installed, several thin wires are coiled together to form athick wire which will not the case with DC.

17.8 DISTRIBUTION OF ELECTRICAL ENERGY FORHOUSEHOLD PURPOSES

You may have seen huge electricity poles, transformers, wires etc. around yourhouses. The production of electricity is done far away from cities at electricitygeneration centers. These power plants depend upon water, thermal energy, windor geothermal power. Here electricity is produced usually at 11 KV (voltage), 50Hertz (frequency). The system by which electricity is transmitted from such centersto the consumer can be divided into two parts.

A) Transmission system.

B) Distribution system.

By using step up transformer voltage is converted transmissions of electricity at theproduction centre. At the electricity generation centre, by using step up transformervoltage is converted from 11 kV to 132 kV. Then the electricity reaches at low powerstation through high tension wire. At lower power station it again converted up to3.3 kV by using step down transformer. In this way by using step down transformerelectricity reaches at home at the village of 220 V and 50 Hz. Hertz (Hz) is unitof frequency. The number of cycles completed by AC in one second is called asits frequency. A frequency of 50 Hertz means, AC completes 50 cycles per second.That means AC flows in one direction 50 times while in the other again 50 timesin electrical wires, bulbs and other electrical appliances. This means that a bulb lightsup 100 times and goes out 100 times in a second. But due to the lack of perceptionof such small intervals of time, a bulb appears to glow constantly.

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Fig. 17.16 Distribution of electrical energy

If the voltage of a transformer is increased then current flow reduces in the sameproportion. Thus by using step up transformerswe change electricity to higher voltage andreduce the current flow. By transmitting thislow current we reduced the losses occuredduring the transmission.

The distribution system is the arrangementwhich provides power from substation to theconsumer. It involved feeder distrinbutors,sub distributors and service men.Normallythere are two types of distribution systems.

1. Tree system 2. Ring system

These days mostly, ring system is used. The arrangement of various component ofrings system made distribution is shown in Fig. 17.17.

17.8.1 Household Circuits

Till the poles near our houses, electricity reaches through the distribution system. Twowires from the poles come to our houses. Among these one wire is called as ‘phase’while the other is called as neutral. In the phase wire the voltage is 220V while inthe neutral the voltage is zero same as that of earth. It is represented as N. Usuallythe phase wire has a red coloured insulation over it while neutral wire has insulationof any colour other than red or green. Inside the houses wiring is done in parallelmode such that when one lights an appliance in a room it doesn’t affect the strengthof current in another room.

Household circuits are shown in Fig. 17.18. We use another wire that has greencoloured insulation over it which is called as earth wire or earth-connecting wire.All the appliances are connected to this to the earth.

Fig. 17.17 Ring System

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Fig. 17.18 Household circuit (one bulb, one fan and one plug point)

This electrical energy is produced by using our natural resources. Population growthgrowing urbanization is increasing the demand of electricity day by day. This iscreating a pressure on our natural resources. Thus it is important today that we useelectricity judiciously and not waste it in any way.

17.8.2 Precautions to be taken while using Electrical Energy

If electricity is used in a careful and safe manner it is the largest and most convenientform of energy. If one uses it carelessly it will become lethal.

1. Before working with electricity one must ensure whether it is AC or DC current.DC of the same voltage of AC is more dangerous.

2. Do not touch electricity supply wires with your bare hands. One may even diedue to shock from current. AC flungs a person away while one gets stuck toa DC source. The main switch should be switched off in case of any accident.One must separate a person who has received a current shock with the helpof a safe nonconductor eg. (rubber, stick, shoes, gloves). Never touch such aperson directly.

3. Never use water to extinguish fire caused due to electrical spark.

4. Always ensure that a main supply is switched off before working on electriccircuits. Use rubber gloves, shoes and separator device when it is necessary towork on live circuit.

5. In household wiring always use good quality wires, proper thickness andinsulation. All the materials should be ISI marked. Connector should be tight andjoined should be covered with insulating tapes. Ensure that the safety measuresof earthing and fuse are properly done in your household electrical circuit.

6. Ensure that a miniature circuit board (MCB) is there or at least a fuse wire ofappropriate load capacity is present.

7. All switches can be switched off by simply closing the single large main switchso that current flow to all appliances is cut off in the emergency.

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17.8.3 Accidents Caused by Electricity

You may have often heard that several dangerous accidents have occurred due toelectricity at homes or industries. Such accidents by electricity occur due to thefollowing reasons;

� leakage of current

� short circuit

� over load.

1. Leakage of Current

Often due to continuous flow of electric current the insulation over wires gets affectedand is scraped off and the wires are left bare. Current leakage occur through suchbare wires. Often these bare wires in contact with a metallic surface increase itsvoltage to that of the main source. The surface of the metal if comes in contact withearth, allows current flow into the earth. When a person touches such appliancesgets a severe shock.

2. Short Circuit

If somehow the main and neutral wire come in contact with each other there is asudden huge spark that takes the form of fire.

3. Overload

If several appliances are connected to the same circuit there is an overload in thecircuit. The value of current flow goes above the required value of the circuit. Atthis juncture the wire fails to bear the load of electric current. This is calledoverloading. Household appliances are connected parallel to each other in the circuit.The greater the amount of resistance the source would take more of current. Youmay have seen that during summer when the demand on electricity increases,transformers often burn due to extra load.

17.8.4 Safety Devices used in Electrical Circuits

1. Electrical Fuse

A piece of wire made of lead and tin alloy is used in making fuse. It have its meltingpoint lower and high resistance then that of electrical wire. Due to this, if currentin a circuit increase above a particular point the fuse wire gets heated and burns out.Due to this the whole circuit is saved from burning. The fuse wire is connected tothe main source in series. Usually 5 A (ampere) fuse is used for household appliances,

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414

while 15 A (ampere) fuse is use for power circuits. 15A fuse wires are thicker than5A (ampere) fuse wires.

2. Miniature Circuit Breaker (MCB)

These days MCB is attached to the household circuit wirings. MCB is a selfregulatory switch which saves the circuit from overloading as well as from shortcircuits. If there is any barrier in the flow of electrical current it immediately stopsthe flow of current. Fuse is also used for this purpose but MCB is prepared in differentshapes varying in use from small to large appliances saving them from high voltage.

3. Earthing of Electrical Appliances

Leakage of electric current in electrical appliances can harm us and may get electricalshock by touching them. Thus as a precaution there is another wire other than phaseand neutral which is called as earth wire. The metallic end of all appliances isconnected to one end of this wire and the other end is attached to a copper plateand burried deep in the ground. Thus, the body of all electrical appliances is of thesame potential difference as that of the earth.

If ever we come in contact with electrical current, the path of earthing would beshorter than that through our bodies and thus we would be saved as current wouldflow through the alternative (earth) pathway rather than through our bodies.


1. The work of an electric generator is to:

(i) Change chemical energy to electrical energy.

(ii) Change mechanical energy to electrical energy.

(iii) Change electrical energy to mechanical.

(iv) Change electrical energy to chemical energy.

2. Appliance that works on the principle of electromagnetic induction.

(i) Electric kettle (ii) Electric bell

(iii) Electric lamp (iv) Electric generator

3. Electric fuse should have the following combination of melting point andresistance:

(i) high resistance and low melting point

(ii) low resistance and high melting point

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(iii) high resistance and high melting point

(iv) Low resistance and low melting point

4. Which principle states that by changing magnetic field, current produced:

(i) Coulombs law (ii) Magnetic behaviour of a solenoid

(iii) Electromagnetic induction (iv) Ohms law

5. Appliance that converts high voltage to low voltage is:

(i) Step up transformer (ii) Step down transformer

(iii) Rectifier (iv) Amplifier

6. Fuse wire is made up of

(i) Silicon and Tin alloy (ii) Tin coated with zinc

(iii) tin coated with nickel (iv) Tin coated with aluminium

7. According to Flemmings left hand rule the force acting on the current carryingconductor is:

(i) Parallel to magnetic field and current flow.

(ii) Perpendicular to magnetic field and current flow.

(iii) Parallel to magnetic field but perpendicular to the direction of current flow.

(iv) Perpendicular to magnetic field but parallel to direction of current flow.

8. Which wire saves appliances from damage among those that come into ourhomes?

(i) Phase (ii) Neutral

(iii) Earth (iv) None of the above.

9. Give the three reason of accident caused by electricity.

10. Name one of the tool used to check the current in wire and commonly usedin home and industry.

11. Sometimes while touching the electrical appliance we have a shocked. Whatwill the commonly answer for this?

12. Two coils A and B are placed close to each other. If the current in the coil Ais changed, will some current be induced in the coil B? Give reason.


� Magnetic field is that region around a magnet that deflects a magnetic needle keptin the region due to application of force.

� The electrical appliances like fan, mixer, juicer-grinder, crane etc. that use motorsare based on magnetic effects of electric current.

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� A change in magnetic lines of force present in conjunction with a current carryingcoil produce electric current. This is called as electromagnetic induction.

� The strength of an electromagnet depends upon:- (i) the current that flowsthrough it; (ii) No. of magnetic lines of force in unit length of coil; (iii) Natureof core etc.

� Flemings left hand rule gives the direction of force that acts upon a current carryingconductor placed in a magnetic field.

� Electrical generators are such devices in which mechanical energy is convertedto electrical energy.

� Electric current is transmitted at high voltage and low current from one place toanother.

� Rectifiers are used to convert AC to DC. Current produced by a battery is DC.

� Household appliances are always connected in parallel so that if the applianceis used, it does not affect electric current taken by other appliances.

� Transformers convert high voltage to lower (step down transformers) or lowvoltage to higher voltage (step-up transformers).

� Fuse wire has a low melting point and high resistance.

� One must wear rubber shoes and gloves while working with electrical circuits.It is because rubber is a bad conductor of electricity. Thus current does not flowthrough it.

� During an accident due to electricity switch off the main switch. The person whois a victim of electric shock should be separated from the appliance or lifted fromthe ground with the help of a non-conductor. The person should not be touchedin any case.

� During disaster like fire or earthquake try to switch off the main switch.

TERMINAL EXERCISE

1. Why does a compass needle get deflected when brought near a bar magnet?

2. Explain magntic field using the concept for magnetic line of forces.

3. Write down the properties of magnetic lines of force.

4. Explain the force acting on current carrying conductor in a magnetic field.

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5. How is an electromagnet made from a solenoid? Explain. Write down thedifferences between bar magnet and electromagnet.

6. What is electromagnetic induction. Explain in detail the functioning of any oneappliance based on this principle.

7. Describe the advantages of AC over DC.

8. What is the function of an earth wire? Why is it necessary to earth electricalappliances.

9. Make such a household circuit that shows current coming from a pole to theroom and at least a fan and a bulb can be lit. Explain the use of socket, switchand fuse as well.

10. Name some devices in which elctric motors are used.

11. How does current reach from electricity production centre to the houses?

12. Explain the structure and functions of AC generator.

13. Explain the magnetic effects of electric current and on the basis of this explainthe functioning of the electric bell.

14. What is Flemings left hand rule?

15. When does an electric short circuit occur?


17.1

2. it properties does not changed

3. speaker in handset

4. North-South

5. yes, but their strength depends on where you are

6. (ii) South pole, which corresponds to the geographic north

7. Magnetic poles are the surfaces from which the invisible line of magnetic fieldemanate and connect on return to the magnet

17.2

1. (i) 2. (ii)

3. (ii) 4. (i)

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5. (i) Number of turns

(ii) Current flowing in the coil

(iii) Length between the poles

6. A solenoid is used for making electromagnets. The use of soft iron rod as corein a solenoid produces strong magnetism.

17.3

1. (ii) 2. (iv) 3. (i) 4. (iii)

5. (ii) 6. (i) 7. (ii) 8. (iii)

9. Leak of current, short circuit, overloading

10. Electrical tester

11. Proper earthing is not there

12. When the current in coil A is changed the magnetic field associated will alsochanges. As a result magnetic field around coil B also changes. This changesin magnetic field lines around coil B induced an electric current. This is calledelectromagnetic induction.

Notes

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Sound and Communication


18

SOUND AND COMMUNICATION

In our daily life we have conversation amongst ourselves. We hear the chirping ofthe birds or horn of the vehicles or mewing of the cat. They are of so many types,so many tones, and so many levels of loudness. In fact usually we can recognizea person by just his or her voice.

We communicate in many ways. Even an infant communicates and does it just withsound, expressions and without words. Adults communicate by talking or writing toeach other. Most often it is our voice that enables us to communicate, whether directlyor through phone. Even an illiterate person can speak. While talking directly is themost used form of communication, technology has enabled us to use many other wayslike telephone, radio, television, text message like paging and sms, and internet. Thedirect communication and use of telephone, satellite etc. differ in the waves used forsending sound. All make use of waves, but we use sound waves (which aremechanical waves) in talking while electromagnetic radio waves in sending voicethrough the radio set or telephone.

OBJECTIVES

After completing this lesson, the learner will be able to:

� describe the characteristics and nature of the wave;

� distinguish different types of waves- the mechanical (sound) and theelectromagnetic waves;

� explain the uses of different kinds of waves; use in communication devices(SONAR and RADAR);

� describe the need and importance of communication;

� identify and appreciate different type of communication systems and

� highlight the use of computers and satellites in communication.

Notes


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18.1 CHARACTERISTICS AND NATURE OF THE WAVE

Sound is a result of vibration. The vibration is produced by a source, travels in themedium as a wave and is ultimately sensed through the ear - drum. Let’s try tounderstand it better by an activity. We can do a simple experiment to show theassociation between vibration and sound.

ACTIVITY 18.1

Take an aluminum wire (about 30 cm in length) or simply a metallic hanger, suchas of aluminum then bend it so as to shape it like a bow. Take a rubber band oran elastic string of sufficient length. You may also use a twig, a small piece of a tree-branch. Tie a thread or an elastic string such as a rubber band to the ends of thebow such that string remains under tension. Ask your friend to record that:

(i) If you pluck the string, you can hear some sound. You may have to adjust thecurve of the bow to be able to hear the sound. You’d notice that the soundvanishes if you hold the string after plucking. If you look carefully, you can realizethat the sound comes only as long as the string vibrates.

(ii) You can check the vibrations. Take a small paper strip (about a cm in lengthand 2 to 3 mm in breadth), bend it in middle to form a V and place it overthe string. You may try the same with string instruments like guitar, sitar, andektara or even use powder on percussion instruments like tabla, drum or dhol.If you leave a little powder or dust on the tabla, and cause the membrane tovibrate, you may be able to ‘see’ the vibrations. A gentle touch with finger tipswill also tell you that vibrations are associated with sound in all these cases.If you strike a steel tumbler with a spoon, hear the sound and then hold thetumbler with firm hand, the vibrations will cease and so will the sound.

Discuss the observations with your friends. Can you now conclude that the soundhas an association with vibrations? These vibrations are transmitted in a mediummechanically and that is how sound travels. It travels like a wave. A medium is amust for mechanical waves like sound to travel. We speak and expect to beheard. But it will surprise you to learn that without some aid, we can’t converse onMoon as we do here. This is because there is no air on moon (actually there is somebut very little) and sound needs a medium to travel. In contrast, we can receiveelectromagnetic waves from distant stars and artificial satellites in space aselectromagnetic waves need no medium to travel. A wave involves a periodicmotion, movement that repeats itself. It also transports energy. Let’s understandwaves better.

What happens if you throw a stone in a pool of water? You will see a disturbanceof a circular shape moving from the point of fall of the stone outwards. We also

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observe that the disturbance is made up of a raised ring in water which seems totravel outward. Soon there is another similar circular feature originating at the samecentre and moving outward. This goes on for quite some time. Even though thereappears to be a movement of material, actually it is only the position of the disturbancethat is changing. This is a wave and is made up of the raised part (crest) and lowpart (trough). So crest and trough are essential components of a wave. A wavetransfers energy from one point to the other without the medium particlesmoving from one point to the other. Thus wave is clearly different from particle.

Understanding the nature of sound requires observations. We observe a flute playercontinuously shifting fingers over holes to produce different notes while playing a tune.Similarly a sitar player also keeps pressing the string at different points touchingdifferent frets (parda in Hindi). When you strike an empty glass with a spoon andwhen you strike one filled with water, different notes are produced.

The science of sound helps us in understanding the reasons behind such things.Besides, the understanding of sound has enabled scientists to devise gadgets whichare very useful. These include hearing aids, sound instruments like speakers, soundrecording and sound amplifying devices etc.. We shall also learn about varioustechnological tools that have been developed to improve communication. Byimprovement we mean we can communicate to more people, at greater distances,and with more clarity.

18.2 REPRESENTING A WAVE

We need to describe a friend by name, height, colour, gender etc for identifying.Similarly, we have to specify some qualities that we shall call parameters, for wavedescription. A wave is represented in terms of its wavelength, amplitude, frequencyand time period.

Fig.18.1 A representation of wave

18.2.1 Amplitude

The (maximum) height of the wave.

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18.2.2 Wavelength

The distance between adjacent troughs or adjacent crests, measured in unit of lengthsuch as meters and expressed by symbol λ (lambda). For longitudinal wave, it willbe distance between two successive rarefactions or compressions.

18.2.3 Time Period

This defines the time it takes for one complete wave to pass a given point, measuredin seconds (s).

18.2.4 Frequency

The number of complete waves that pass a point in one second, measured inHertz (Hz).

18.2.5 Speed or velocity

Wave speed is defined as the distance travelled by a wave disturbance in one secondand is measured in meters/second (ms–1). Speed is scalar quantity while velocity isa vector quantity.

Not all of these properties are independent; one can relate some. Period is inverselyrelated to the frequency. This means if the frequency is high, the period will be low.This is understandable because frequency is number of times a wave completes aset of up and down movements (or a set of crests and troughs) in 1s. If these occurmore frequently, it has to be done in very short time. Mathematically one may sayperiod

T = 1/n

Where ‘n’ is frequency. We just said that wavelength is equal to the distance betweentwo successive crusts or troughs. In one second this distance is covered a numberof times given by frequency.

So, Velocity = frequency × wavelength

or V = n × λ

The waves that produce a sense of sound for living beings are called sound wavesor audible waves. Only those waves that have frequencies lying in the range of 16Hz to 20,000 Hz are audible to human beings. However, this range is an averageand will slightly vary from individual to individual. Sound waves with frequenciesbelow 16 Hz are called infrasonic waves and those above 20 kHz are ultrasonicwaves. Animals like bats are able to produce and sense waves beyond the rangeof human audibility and use it for ‘seeing’ in the dark.

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18.3 MOVEMENT OF SOUND IN AIR

Sound waves travel in fluids and solids as longitudinal waves. A longitudinal waveis a wave in which vibration or the displacement takes place in the direction of thepropagation of the wave. Sound moves due to difference in pressure. If a sound isproduced in air, it compresses the adjacent molecules. Due to the compression, theair pressure increases. This causes these compressed molecules to move in thedirection of the pressure that is the direction of the wave. But displacement of themolecules causes fall in pressure in the place they left. If the wave is continuing thenanother rush of molecules comes in, fills the empty or rarified space. This processis repeated and the disturbance propagates. Thus a chain of compressions andrarefactions is generated due to sound. They travel and transport energy. If thereis no medium, then produced sound will not be able to push any medium-moleculesand sound will not move. That is the reason why we can’t hear on moon; there isno air in Moon’s atmosphere and sound can’t travel.


1. Which sound wave will have its crests farther apart from each other - a wavewith frequency 100 or a wave with frequency 500?

2. If the velocity of sound is 330 metres per second (ms–1), what will be wavelengthif the frequency is 1000 Hertz?

3. What is the approximate audible range of frequency for humans?

18.4 DIFFERENT TYPES OF WAVES

The waves can be of different types. These may be mechanical or electromagnetic.Mechanical wave is a term used for those waves that require a medium for travelling.Its speed is dependent on the properties of the medium such as inertial and elasticproperties. In other words the speed of the wave will depend on how easy or difficultis it to displace the particles of the medium (that is to say on their inertia) and onhow those particles regain their original positions which is the elasticity.

An electromagnetic wave results from acceleration of charge. It doesn’t require amedium to travel. It can travel through vacuum such as light do waves which travelfrom stars through empty space to reach us. The electromagnetic wave has electricand magnetic fields associated with it. The 2 fields, electric and magnetic, areperpendicular to each other and also to the direction of propagation. When wemention the wave length of an electromagnetic wave, we don’t mean any physicalseparation between any crests or troughs or between rarefactions and compressions.This is because sound wave creates low and high pressure points in traveling through,say, air. But the electromagnetic wave needs no medium so there are no material

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troughs or crests or material rarefactions and compressions. It moves with the velocityof light viz. 2.997925 × 108 m s–1 that is 2.9997925 lac km s–1 in free space.

When thinking about movement of sound wave in a medium, we should alwaysremember that the medium is a collection of particles. Movement of one particle canaffect the other particles. You may have seen bicycles falling when parked in a rowclosely and one of them gets pushed accidentally. When the adjacent bicycles startfalling in sequence, here also we see a wave, a movement of a disturbance. Hereone bicycle imparts energy to the next bicycle, which transfers it to the next and soon. Here also there is a disturbance travelling without the medium component (thebicycles) moving to the end. The Sound wave is a mechanical wave but light waves,infra red rays, X-rays, microwaves, Radio waves etc. are electromagnetic (in shortem). Gamma rays are alos em waves and result from radioactive decay of nucleiof atoms. Compared to sound waves, the em are much more energetic. They travelat the velocity of light that is about 3 lac kms per second in vacuum. In comparison,the sound waves travel very slowly. In air, it travels at 330 m s–1. The velocity ofsound in some media is given in the table which shows that sound moves faster inthe solids than in gases or liquids.

Table 18.1: Velocity of sound in different materials

Medium Velocity

Steel 5200 m/s

Water 1520 m/s

Air 330 m/s

Glass 4540 m/s

Silver 3650 m/s

Such difference in the velocities of light and sound means if there is an event in thesky, which produces light and sound both, we shall see the light almost instantly butit will be a while before we hear it. When there is a lightening in the sky, we seethe light before we hear the sound. Mechanical wave can be either transverse orlongitudinal while the electromagnetic wave is only transverse. The transverse waveis one in which the motion of wave and of the particles are perpendicular to eachother. In a longitudinal wave, the motions are in the same direction. The sound wavecan be of 2 types: Transverse and longitudinal.

We can try to visualize transverse wave by tying one end of a rope to a hook orpeg in a vertical wall (or to a door-handle) and holding the other end such that therope remains loose. We can demonstrate a transverse wave travelling along the ropeif we quickly give up- and down- jerk (or even in horizontal plane) to the rope atour end. We see the wave travelling between our hand and the peg while the points

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on the rope move perpendicular to the rope and wave. This is a transverse wave,as the particles of the medium move perpendicular to the direction of wave movement.In the example of wave when we throw a stone in stationary water in a pond, itis more complex but here we confine to what happens on the surface. We see thaton water surface the wave moves from the centre to the shore. If we see a duckor a small paper boat there, it oscillates up and down with water that is goes uptemporarily after which they come back to their mean positions without shifting theposition horizontally. That makes it a transverse wave.

In a longitudinal wave, the displacement of the particles and propagation of thewave are in the same direction. For instance, if we blow a horn, speak, or quicklymove an object in air we are pushing the air molecules. These molecules, in turn,push the adjacent molecules which impart their energy to the next ones. After losingenergy in the interaction, the molecule is back to its original (mean) position. Thisresults in formation of compressions and rarefactions. So it’s the compression (orrarefaction) which is travelling and not the molecules. Just like the distance betweentwo successive crests or troughs is a measure of wavelength for transverse waves,the distance between two successive compressions or rarefactions is termedwavelength of the longitudinal wave.

Fig. 18.2 Formation of rarefactions and compressions inair and indication of wavelength

While transverse waves form only in fluids (air and liquid), the longitudinal wavescan form in all the three media viz. solid, liquid and gas. One way to visualize alongitudinal wave is to take a spring, fix it between two ends and then pull or pressit on one end along the length. Compressions and rarefactions can be seen movingand rebounding along the axis of the spring.


1. Does a wave transfer energy or material?

2. How do mechanical and electromagnetic waves differ?

3. What is the difference between a transverse and longitudinal wave?

4. Do transverse waves form in solid?

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X-Rays

Gammarays

Wavelength (cm)

10–11 10–9 10–7 10–5 10–3 10–1 10 103

Radio

The electromagnetic spectrum

MicrowaveVisible

InfraredUltraviolet

Table 18.2: Ranges of wavelengths and frequencies ofelectromagnetic radiations.

Name Wavelength Wavelength Range of Energy (eV)(Angstroms) (centimeters) frequency (Hz)

Radio >109 >10 <3×109 <10-5

Microwave 109-106 10-0.01 3×109-3×1012 10-5-0.01

Infrared 106-7000 0.01-7×10–5 3×1012-4.3×1014 0.01-2

Visible 7000-4000 7×10-5-4×10–5 4.3×1014-7.5×1014 2-3

Ultraviolet 4000-10 4×10-5-10–7 7.5×1014-3×1017 3-103

X-rays 10-0.1 10–7-10–9 3×1017-3×1019 103-105

Gamma rays <0.1 <10–9 >3×1019 >105

Fig. 18.3 Various radiations with wavelength and frequency

18.5 NATURE, MEASURE AND QUALITY OF SOUND

Sound level is measured in units of decibel (dB). Here deci means one- tenth andbel is the level of sound. The term Bel is after the name of inventor of telephoneAlexander Graham Bel. Actually it’s a unit which compares the levels of power oftwo sources. Two power levels P1 and P2 are known to differ by n decibel if

n = 10 log10 P2/P1

Here log10 means log with 10 (and not e) as base. Here P2 is the sound which ismeasured while P1 is a reference. Normally, the reference is a sound which is justaudible. For average human ears, the whisper is about 30 decibel. The normalconversation is about 65 decibels while a jet plane taking off makes a noise of about150 decibel. Beyond 85 decibels, sound is damaging and can lead to temporary lossof hearing. Prolonged exposure to noise can cause permanent hearing loss. We mustbe careful not to cause noise even when it means celebration for us. So it is notadvisable to play band in a marriage procession (barat) near hospitals where patientscan suffer because of noise. Noise raises the blood pressure and causes anxiety. Even

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if we don’t realize, constant noise causes tension. Crackers during festivals are alsoharmful as they not only pollute air but also create noise.

Considering the effect of sound on human health, it becomes necessary to developan instrument to measure loudness of sound. The Decibel meter makes use ofa special crystal called Piezo electric crystal. This has a quality that when subjectedto pressure, it generates electrical voltage. In a Decibel Meter, a combination ofa mic and piezoelectric crystal is used. Sound causes the diaphragm to vibrateand press the crystal and an electrical voltage generated is measured giving anidea of the sound level. This voltage can be converted into digits using calibrationand displayed. Thus one can estimate noise from fire crackers, vehicles andmachines and monitor so that people are not exposed to noise above certain level.The fact is that even music played at a very high level can cause deafness if donefor a long time.

Different sources sound different. We should not confuse between loudness and pitch.Sound from a metallic tumbler on hitting with a metallic spoon is higher in pitch thansound from a pitcher when hit with awooden stick. The voice of females isgenerally higher in frequency than malevoice. However, we should also knowthat voice is not just one frequency. It isa mix of many frequencies, some ofwhich are multiples (called harmonics)of the same frequency called fundamentalnote for the person.

Now, that we know the relationshipbetween wavelength and frequency, wecan appreciate why a flute produces ahigher pitch (smaller wavelength, largerfrequency) when all holes are open. When all holes are closed, it produces the largestwavelength. Actually, the clue lies in relationship v ∝ 1/ λ and by blowing harderwe can produce louder notes.

ACTIVITY 18.2

You can do a simple experiment to understand the relation between the pitch of asound and the wave length. This will help us to understand difference in sounds froma dhol and tabla as well as from a small and a long string. The smaller one will producea higher frequency.

Fig.18.4. A musical string instrument(which you can make using a

metallic box and metallicstrings)

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Take a hollow preferably metallic box such as of toffees or sweets. If unavailable,you may use a ply-board or cardboard box. Take 3 pieces of metallic strings (wire)available from a musical instrument shop or get from a hardware shop. Nails or nutsand bolts (also available from the hardware shop)

The nails/bolts have to be fixed on the box. Hammer the nails to fix them on thebox/board using adhesive if needed. Alternatively, drill holes in the board (or top ofthe box) and using nuts, fix bolts as 3 sets of 2 each. As shown in the diagram, thedistance between nails in the sets should be different. For instance, if the 2 nails/bolts are spaced by 10 cms, make it 20 and 30 cms in the other sets. Now metallicstrings, should be stretched one each between 2 nails/ bolts in each of the three sets.

If you pluck the string, you can hear some sound. For each length of string, the soundwill be different. The shorter string will produce higher frequency.

Invite a group of friends for a show of this home made instrument. All of you canobserve that when you pluck the strings, the pitch of each of the three strings isdifferent. The longer string will allow a longer wavelength to be set up and hencehave the shorter frequency (remember that higher wave length means the lowerfrequency for the same velocity). This is also the principle on which sitar and othersuch string instruments work. The frequency also depends on the tension in the stringwhich is vibrating. This may be verified with a simple experiment. You may vary thetension in the strings by rotating the bolts or slightly bending the base board if it’sflexible. It may be also achieved by passing the string over a metallic base (whileone end remains fixed to nail/bolt) and suspending different weights from the otherend of the string one by one. You may also fill water in similar glass or steel tumblersbut to different levels. When you strike them with spoon, the sounds in them willbe different in pitch. The pitch or the frequency will be higher where the air pipeis shorter (water level is higher). The wave length of the sound produced will beproportional to the air column lengths. Which of the 2 in a set of 2 tablas generatehigher frequency? Is it the one with smaller diaphragm or bigger? A bigger diaphragmallows bigger wavelength to be set up.

Fig.18.5 Graphical representation of changes in sound pressurewith time in musical and noisy sound

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Music is a set of sound that is pleasing to hear and is not random. It refers to thequality of sound as well as the tune. Noise is random and irritates while music hasperiodicity whether in beats, or rhythm. For instance, in a song, you’d notice thatthe same tune is repeated after certain period. After a stanza, the singer comes backto the same tune (combination of notes). If we plot sound pressure with time, we’dnotice that it is sweet if it changes in a regular fashion. Noise, in contrast, changesin an irregular fashion and irritates. Sound is evaluated by musicians in 3 terms: quality,pitch and loudness. Two sounds may have the same loudness, may be at the samepitch but can still have different quality/timbre. That is how we can distinguish thesounds from Sitar and guitar even when the loudness and the pitch are the same.

ACTIVITY 18.3

Take a flute and close all the 6 holes of the flute with your fingers (don’t use smallestfingers). Blow in to the flute and hear the sound. Now keeping the same positionsof the fingers (that is keeping all the holes closed), blow harder. You’d hear a loudersound. If we blow intermittently the sound you hear may be unpleasant but whenyou blow continuously you can hear a pleasant sound.

In India we see many musical instruments. Flute (Baansuri), Sitar, Sarod, Tabla, drumghatam (pitcher), and even some Western instruments like guitar, piano andharmonium are quite popular. Some are string instruments where sound is producedby plucking a string and setting it to vibrate such as Tanpura, sitar and ektara. Somelike tabla and dholak are percussion instrument where a membrane is made to vibrateby striking with hand or a stick. Then we also have flute and trumpet where we blowair into a pipe to produce sound.

Tanpura Sarod Sitar

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A set of Tablas Drum Flute

Trumpet Dholak

Fig. 18.6 Some musical instruments

Flute is believed to be the oldest musical instrument. A flute made with bone froma vulture’s wing was found in Ulm (South West Germany) in 2008. It had only5 holes while modern flutes have 6 or more. It was dated to be about 35,000year-old.


1. What is the unit to measure sound intensity?

2. Why do they have many holes at the side of a flute lengthwise ?

18.6 USES OF DIFFERENT KINDS OF WAVES INCOMMUNICATION DEVICES (SONAR ANDRADAR)

SONAR is a technique that makes use of this property of sound. SONAR standsfor SOund NAvigation and Ranging. This works on the principle of echo oftransmitted sound waves from objects. For instance, if you hit a wall in front witha tennis ball, the ball will bounce back to you. But if the wall is removed, the ballwill not come back to you. Thus even with eyes closed, you have a way of knowingwhether there is an object or a rebounding surface in front. SONAR works in thesame fashion.

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Use of sound waves to detect objects is based on the above simple example. Theadvantage of using sonar wave over electromagnetic waves is that electromagneticwaves lose energy fast in the ocean water because water can conduct electricity.In contrast sonar waves can travel farther in water.

Fig. 18.7 Working of SONAR: The continuous lines are used for transmitted SONARwaves and the dashed lines are used for reflected SONAR waves.

There can be two types of SONAR set-ups. One is Passive and the other Active.In Passive SONAR, one detects sound waves that are present around. Leonardoda Vinci did it as early as 1490 AD. He dipped a pipe in water and placed his earnext to the end which was out of water. He used this to detect the waves generatedby ships. Today, the techniques are far more sophisticated. SONAR became a topicof very serious studies during the World War II as detection of movements of shipsand submarines assumed significance.

Have you ever been to a valley and shouted or clapped to hear echo (or echoes)of your voice or of clapping sound? The echo comes from the hills. It's a reflectionof your voice, or of the sound produced by you. Even in a very huge hall orbetween two fairly distant walls or building, one can hear echo. When thereflection is from a far away object, we can distinguish it as an echo. But if thereflection is from a close by object, it is perceived by mind to be a part of theoriginal sound. The reason most of the people find their voice so pleasing to hearin the bathroom is that there the echo comes quite early, such that it is joinedto the original sound. We can discern a sound in time if it is separated by morethan 0.1 second. A bathroom is usually too small and the reflected sound comesback well within 0.1 second.

Now Active SONAR is very important. It has two major components:

1. A transmitter consisting of a signal generator, power amplifier and a transducer

2. A detector which may be a single detector or an array of several detectors

Object ortarget

Transmitter andReceiver of

SONAR waves

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One has to ensure that the signal is sent as a narrow beam. If not, then the reflectionswill be coming from many directions and will be confusing. Theoretically, the distancetravelled by the wave is twice the distance between the transmitter/detector and thetarget to be detected. So if velocity of the sound in water is v, then distance of theobject

d =1

2 × v × t

where 't' is the time-lapse between transmission and detection of sonic signal.

The wave may be reflected from surface or bottom of the sea, ships, whales or otheranimals, submarines and other objects. The whole thing looks very simple but inpractice, there are several other factors to be considered. For instance, the velocityof sound in a medium depends on the density and bulk modulus of the medium.

18.6.1 Radar

RADAR is an acronym for RAdio Detection And Ranging and is useful in many waysto us:

1. Observation of atmospheric objects and phenomena like clouds, cyclones, raindrops etc. and prediction of weather

2. Air Traffic Control

3. Ship navigation

4. In military use (early warning and fighter control radar)

RADAR is radio wave equivalent of SONAR. In RADAR, a radio wave does thesame job as sound wave in SONAR.

The basic elements of RADAR system are:

1. A pulse source and a transmitter with an aerial which’d emit radio waves

2. An object which’d reflect the radio wave

3. A receiver with an antenna and a display system like Cathode Ray Tube(somewhat like a television or a computer monitor)

Fig. 18.8 A simple sketch of components of RADAR

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Transmitter: The transmitter in a RADAR system generates and sends a radio wave.The radio waves go in all directions. If there is any object, the wave is reflected byit. There has to be a receiver to detect the reflected wave. The radio waves areelectromagnetic radiation and travels at the velocity of light. It’s obvious that the timegap between the outgoing radio wave and arrival of the reflected wave is very small.So what is done is that as soon as a radio wave is emitted, the transmitter is switchedoff and the receiver is switched on. Thus the reflected wave is not masked by theemitted wave and even a weak reflected wave is not missed by the receiver. If, aftercertain gap, there is no reflection received, we can presume that there is no objectwithin certain distance and we can switch off the receiver and switch on thetransmitter. This process goes on as was the case with SONAR. This is called apulsed transmitter. However, for detecting moving objects, one can use continuouswave transmitter. If an object is moving away, the frequency of the reflected wavewill be lower than the transmitted wave. If the object is moving closer, the frequencywill be higher for the reflected wave. This is called Doppler Effect for sound. Onecan always adjust the receiver such that it doesn’t receive the radio waves of thefrequency emitted, but receives the radio waves of lower or higher frequencies. CalledDoppler RADAR, such RADAR can only detect a moving object because it can’treceive the frequency at which a radio wave was transmitted and only a moving objectwill change the frequency of the reflected wave.

RADAR is useful in air traffic control as it can ‘see’ in the dark. RADAR can monitormovement of clouds, detect rain drops. It can also detect presence of distant shipsand big animals like whale in the sea. It can also be used to estimate the speed ofthe object approaching or moving away from us. It is used by weather scientists totrack storms, tornadoes and hurricanes. Space and earth scientists make its use intracking satellites and also in mapping earth surface. In fact it is useful even in makingauto-open doors in shops or airports. This is because a wave will be reflected andsensed only if there is an obstacle in the way of emitted radio wave.

18.7 NEED AND IMPORTANCE OF COMMUNICATIONMany of our actions are in relation with actions, expectations or thoughts of others.The same is true of others. Communication need not always be verbal thoughsometimes facial expressions or body language give clues to what is going on in mind.But that is not very common and, generally, thoughts are in mind and we can’t readthem. Have you ever seen a face that conveyed sadness as if seeking help, you tookpity and did something for that person? Possibly yes. But unless you talked, youwouldn't know the requirement exactly. It’s by communicating among ourselves thatwe know each other’s thoughts and take actions. Therefore, communication is veryimportant in life and an illiterate person doesn’t read or write, communication throughsound assumes prime place. Sometimes, sound is heard directly, sometimes throughinstruments like loud speakers and sometimes it's communicated over large distancesusing complex equipments.

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18.7.1 Different type of communication systems and devices

Some common devices for sound communication, other than independent oral orprinted communication, are given below:

(i) Microphone and speakers

(ii) Telephone

(iii) Satellite, Computer and internet in communication

(iv) HAM

(i) Microphone and speakers

ACTIVITY 18.4

To understand that air pushes when in motion, take a candlestick, matchbox, a fanand a loudspeaker. Light the candlestick and hold it in front of a running fan. Theflame flickers, and sometimes extinguishes. Air in motion has pushing quality. If youdo the same exercise with a burning candle and a loud speaker, a similar thinghappens. The reason for the flame getting extinguished is the pressure of the air. Inthe first case the source is the fan, in the other it is the loud speaker. A loud speakerreproduces sound through the motion of its diaphragm which pushes the air, leadingto compressions and rarefactions.

The microphones (mic or mike in short) and the speakers are very commonequipments. You see them not only in public meetings and conferences, you comeacross them even when you use your phone. The work of a microphone and aspeaker are opposite of each other. A microphone converts sound into electrical entity(voltage) while a speaker converts the voltage into sound by moving the diaphragmof the speaker and producing vibrations in the air. Basically a microphone has adiaphragm which moves when soundpressure pushes it. This movement canbe converted into proportional voltageusing several possible transducers. Herea transducer is a device which receiveselectrical, mechanical or acoustic wavesfrom one medium and converts theminto related waves for a similar ordifferent medium.

The microphones can be of severaltypes such as electrostatic (condenser/capacitor using plain or RF voltage), piezo-electric (crystal and ceramic), contact resistance (carbon), and magnetic (moving coiland ribbon).

Fig. 18.9 Symbol of a microphone

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Fig. 18.10 (a) Structure of a condenser (capacitor) microphone

The diagram shows a condenser microphone. It has a thin diaphragm of thickness1 to 10 micrometers. One micrometer (or micron) is one millionth of a meter or onethousandth of a millimeter. Close to thisplate (metallic or metalised plastic) standsanother metallic plate with holes. These2 plates act as electrodes and are keptat opposite polarities by supplyingvoltages from –60 to +60 Volts (DC). Tobehave as a condenser, they should beinsulated from each other. When soundwave pushes the diaphragm, it vibratesand the capacitance of the condenser (orcapacitor) changes. This is because thecapacitance is proportional to the potentialdifference and inversely proportional tothe separation between the plates. Anychange in the separation changes thecapacitance. The capacitance is alsodependent upon the medium but as themedium here remains the same, so weignore this parameter. The values of theresistance and the capacitance are chosen such that the change in voltage isimmediately reflected in the voltage across the resistance in series. Any change inthe capacitance (meaning any change in sound) leads to voltage change. The voltageis fed to an amplifier. When the amplified voltage is applied to the coil of a speaker,it reproduces the sound which changes with the input- sound. The functioning of thespeaker is just reverse. There, electrical voltage is fed to the speaker coil and thechange causes the diaphragm to vibrate and produce sound.

Fig. 18.10 (b) Condenser microphone

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In a ribbon microphone, a corrugated ribbon made of a metal is suspended in amagnetic field. Sound causes the ribbon to vibrate. This means change in magneticflux through the ribbon. This induces an electric current which drives a speaker. Whenthis current is flown through a coil attaches to diaphragm of a speaker, the diaphragmvibrates and produces sound. Special materials developed using nano technologiesare being used to make ribbons that will be light but strong. Being light improvesthe response to sound. The ribbon microphone senses pressure- gradient and notjust pressure. Therefore, it detects sound from both sides.


1. Give three examples of devices that make use of microphones or speakers orboth.

2. In a condenser microphone, what will happen if the diaphragm is made veryheavy?

(ii) Telephone

Invention of the telephone is credited to Alexander Graham Bell. The telephones areof several types: hand sets, mobile phone, satellite phone and through internet. Thebasic function of a phone is toallow communication of voiceboth ways. Of late, phoneswith facility of transmittingimages have also becomeavailable. The telephone maybe with or without wire. In awired phone, the basic structureis as follows. It has amicrophone and a speaker. Themicrophone receives our voiceand converts it into electricalsignal. Similar process occursinside the mouthpiece of thetelephone. A basic telephonehas 3 main parts:

(i) Cradle with a hook switch.

(ii) A mouth piece which houses a microphone

(iii) A hearing piece which houses a speaker (usually an 8 ohm speaker)

The phone is rested on the hooks. As soon as the phone is lifted, the hooks popup and a connection gets made inside the body of the cradle which completes a dial

Fig. 18.11 (a) The basic structure of a wired phoneset (In actual phone there is a provision so that

your voice does not reach and disturb you)

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tone ringer circuit. This produces a tone which is actually a mix of 2 tones (2frequencies). On hearing the dial tone, we know that the phone is connected andwe can dial a number. If the number is busy when we dial, we hear another mixof tones. Over the times, the telephone has undergone many changes includingintroduction of the cordless (for short range) and mobile phones. But as far as basicstructure of a telephone set is concerned, it has remained the same.

Now the dialing is by pressing the keys. We speak into the mouth piece and hearthe other person through the speaker. In a basic phone, the speaker and themicrophones form the ends of the phone set. In this way, the set can be held closeto face such that the speaker is close to our ear and the mic to our mouth.

The speech is controlled by a mouth piece which contains a mic. It includes adiaphragm. In the old phones, the diaphragm was made of a 2 metallic sheets betweenwhich carbon granules were filled. As one speaks, the diaphragm gets pressedfollowing the same pattern as the sound of a speaker. In turn, the carbon granulesalso get compressed and decompressed, coming closer and moving away, thusincreasing and decreasing the conductivity. A current is sent through the diaphragm.The source of this DC current (a few mA) is a battery at telephone exchange andthe current comes to our phone. This leads to varying electrical current. This currentwill depend upon the sound pressure hence this can pattern the signal being sentthrough the amplifier and the cable. Now-a-days, there are electronic microphones.This signal (as electrical current) is sent to a junction box outside house using a pairor copper or aluminum wires. There are signals from other houses also reaching thisjunction box. All of these electrical wires carry voice- signals (sound converted intoelectrical signal) that are sent through a common coaxial cable, housing many pairsof copper wires, to the telephone company’s exchange. From there, they can berouted either through metallic or fiber optical cables. These days, the signals are alsorouted from the exchange through microwaves using satellites especially for internationalcalls. To avoid our own voice reaching our ears, a duplex coil is placed in the circuitof the microphone. In addition, there is a ringer. When someone calls, it rings a belland we know that we have to attend a phone call.

The hearing is controlled by a speaker. It consists of a diaphragm with a permanentmagnet attached to it on one end and an electromagnet close to the other end. Theelectromagnet is a piece of soft iron with a coil wound around it. The signal comesand flows through the coil. This causes the iron core to be magnetized. This naturallycauses the diaphragm to vibrate in the same pattern as incoming current (voice). Thisgenerates sound that we hear.

Mobile phones have brought great convenience in daily life. The basic workingprinciple remains the same in mobile phones also. But for them, the sound doesn’ttravel through cables or wires. It travels as electromagnetic wave through space viaantennas, base towers, switching stations (or even satellite) and then again the

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antenna. When a number is dialed, the (electromagnetic)field is spread all around through antenna of the mobile.The signal is received by the nearby microwave towerand then by the switching station. This station re-transmitsit in all directions (it doesn’t know where the intendedmobile may be) and a part is available to the otherantennas in other places. When an antenna near theintended receiver gets the signal, it also retransmits it andthis is received by the antenna of the intended mobile,which rings.

While fully conclusive evidence may not be available,there are apprehensions about the possible health hazards,such as brain tumour, associated with use of mobilephone. The microwaves, used by mobile phones set-ups,are absorbable by water. It is apprehended by some that prolonged conversationon mobile phone can result is considerable microwave dose to brains which containsfluids. The children’s brains contain more fluids and the skull is thinner. Hence theyare more susceptible. Experimental evidence exists and common experience suggeststhat long duration conversations lead to temperature increase in body part close tothe mobile phone. A study conducted with the support of World Health Organisationby the International Agency for Research on Cancer, catagorises the radiofrequencyelectromagnetic fields as group B agents that could possibly be ‘carcinogenic tohumans’. It may be advisable not to hold them too close to head. One should limitthe use of mobiles to the shortest possible durations especially at a stretch and closeto the same ear. One may also be advised to use earphones if long duration talkbecomes necessary.

(iii) Use of satellites, computers, and internet in communication

(a) Satellites

Satellites are bodies that revolve around planets. All the planets in Solar System,except Mercury, have natural satellites. Moon is a natural satellite of Earth. But wehave artificial satellites launched by several countries. You may have some timesnoticed, after sun- set, tiny points of light moving in the low sky. They are movingtoo fast to be stars. These are artificial satellites glowing due to scattered Sun lightwhich is below our horizon by that time. The first artificial satellite was by nameSputinik-1 and launched by USSR on October 4, 1957. It carried a radio transmitter.The first American satellite to relay communications was Project Score in 1958. Indialaunched its first artificial satellite ‘Aryabhata’ from a USSR launching facility on 19thApril, 1975. This was followed by Bhaskara-I (7th June, 1979). After developingindigenous launch vehicle SLV-3, India launched 35 kg Rohini-I satellite (18th July,1980) using a 4- stage SLV-3 vehicle followed by 2 more in the Rohini series. The

Fig. 18.11 (b) Mobile

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next was Apple (Arianne Passenger Pay Load Experiment). These were followedby many satellites like Bhaskara-II and INSAT (Indian National Satellite) series whichhave been used for communication, TV and radio broadcasts. In 1988, the firstsatellite of IRS series was launched aimed at serious remote sensing work andapplications. Since then, India has successfully launched many satellites for remotesensing and communication.

Having satellites in space places us in a privileged position. If we are on ground thereis a limit up to which Earth’s features can be seen by us. But viewing Earth froma distance has an advantage. It allows us to look at up to half of the planet if thedistance is sufficiently large. We can send electromagnetic signals to the other sideof the globe through the satellites in space. Therefore, the artificial satellites have cometo play a very important role in any country’s infra structure. They serve veryimportant role in communication, space research, survey of natural resources likeminerals on Earth, weather prediction including movement of clouds, also change incourse of rivers, and disaster monitoring (floods, cyclones, tsunami etc.). Communicationbecomes important for imparting education. The idea that satellites could be usedfor communication came from Arthur C Clarke in mid forty’s. That is the reason thegeostationary (or geosynchronous) orbit is also called Clarke orbit. Clarke rose tobecome one of the greatest science fiction writers.

Electromagnetic waves sent from any part of Earth can’t reach just any part of Earth.If sent downwards, they will be limited to a small distance due to curvature of Earth.If transmitted up, they will keep going straight and hit ionosphere, a layer of chargesin the space, at 50 kms and more above the ground. Then they will be reflectedto Earth and reach some part which is far away from the source. Thus a huge areain- between will be a dark zone where the signal wouldn’t reach. Instead of theionosphere, one may use satellites to retransmit the signals. But we need more thanone satellite which can receive the signal sent from ground and re - transmit it indifferent directions. Therefore, it was thought that several satellites in space couldtogether cover whole of Earth and facilitate communication.

A satellite’s position and orbit are critical. The satellite has to be launched using arocket, lifted into the correct orbit and given suitable energy and momentum in theright direction so that it keeps moving. A satellite could be geostationary whichremains stationary with respect to Earth. A satellite in a geostationary orbit keepsmoving at the same angular speed as Earth and in the same direction as rotation ofEarth. So a geostationary satellite has a revolution time which equals the rotation timeof Earth viz. 24 hours. It appears to be in a fixed position to an observer on Earth.It can keep looking at the same spot on earth for a very long time, monitor the changesand transmit the data to ground station. Thus to direct antennas towards the satelliteto receive the signal, one doesn’t have to keep tracking a moving satellite. That wouldhave demanded expensive instruments on ground for direct TV transmission. Thismeans huge savings because it does away with the need for too many ground

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antennas. Placing a satellite at 36,000 kms has added advantage that it just falls underthe gravitational pull of Earth and is energy - economical though it’s more expensiveto launch compared to low orbit satellites. Low orbit satellites are placed about 400kms above ground. But being low, they can see only a small portion of the groundbelow. There are Polar Satellites that move over the poles. The remote sensingsatellites have been placed in comparatively low (less than 1000 km high) orbits incontrast with communication satellites in geostationary satellites which move at36,000 kms above Earth. The remote sensing satellites should be launched such thatthey make observation at any place between 10 AM and 2 PM so that the groundis illuminated from the top and images come out clearer.

A geostationary satellite is useful for countries at low latitude such as India. Thesatellite is placed at an altitude of 36,000 kms, going around in the plane of equatorand making one revolution around Earth in 24 hours. However, as Earth also makesone rotation in 24 hours, the satellite looks at the same place on ground all the time.From this altitude, it can view about one- third of Earth. The signal is sent from groundto the satellite as microwave at certain frequency and the satellite retransmits it tothe other parts of Earth at a different (but still as microwave) frequency. Microwaveis at wavelengths of the order of a millionth of a meter. The highly directional antennason Earth receive these microwave signals. Thus satellites make it possible to sendTV, radio signals to far away places on Earth, even on the other side of globe.

Orbital radius Km

Low Earth Orbit (LEO) 160-1,400

Medium Earth Orbit (MEO) 10-15,000

Geostationary Earth Orbit (GEO) 36,000

Fig. 18.12 A satellite moves in Low (LEO), Geostationary orbit (GEO) or in an orbit.LEO goes over the poles in each revolution (Polar orbit) which is used for

mapping Earth. This is useful in weather studies because itallows looking at clouds etc. at the same time every day.

The geostationary and LEO satellites monitorthe same place on Earth.

(b) Computer and Internet

Today computers are inevitable in daily life. Computers play a major role in publishingindustry; designing of houses, controlling the functioning of cars and garments;computerized machining, regulating air traffic, and in simple as well as the mostsophisticated scientific instruments. Even at home, majority of the gadgets, whethertelevision, automatic washing machine, television or microwave oven, one findsapplication of computers. Besides this, they have revolutionized communication.

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Computers are used for communication to and from aircrafts, ships, and even hugeboats; in money transactions and in maintaining and processing financial records suchas in Automated Teller Machine (ATM) and banks. In the form of application tointernet, computers have emerged as a very strong communication link. Using e-mail,one can send a message, chat live (that is send and receive text) and even talk instantlywhich has revolutionized communication. Earlier it would be weeks before one couldsend a message and receive reply from abroad. Today, it’s a matter of seconds. Thiscertainly helps the dissemination and growth of knowledge.

(iv) Ham

The term HAM is not from the English language. It was coined taking the first lettersfrom the surnames of those 3 persons who started this way of wireless two waycommunication. They were S Hyman, Bob Alby and Poogie Murray. It was in 1908that they started an Amateur Radio Club which has grown to the present worldwidegroup of amateurs. Even today when mobile phones are so common, the HAMcomes handy in case of disasters when all other means of communication break down.HAM uses radio waves. Radio waves are electromagnetic waves in the range (about10 cm to 10 km, see Fig. 18.3) and hence they travel at a velocity (in vacuum) ofabout 3 lac kms per second. Sound is converted into electromagnetic signal andtransmitted using an antenna. The sound is intercepted by the receiver which convertsit back to sound.


1. List some uses of satellites.

2. If a satellite equipped with cameras remains fixed at one height above groundeven as earth rotates and moves in its orbit, what is its possible use?

3. Arrange the low orbit, geostationary and polar satellites in decreasing order ofaltitude above Earth (the highest one comes first).

4. Which of the satellites are preferred for communication application?


� Sound results from vibration and needs a medium to travel, be it gas (like air),solid or liquid. It is faster in solids than in liquids and is the slowest in the gases.

� Electromagnetic radiations also are waves but they can travel through vacuum.

� Wave, sound or electromagnetic, involves periodic movement, movement thatrepeats itself.

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� A wave is described in terms of wavelength, frequency and amplitude. Velocityis equal to the product of wavelength and frequency.

� Noise is random while music is periodic. Music is pleasing to hear but it is alsosubjective. Sustained exposure to noise and even music at high decibel harms.

� The functioning of musical instruments like Tabla, Sitar and flute (Baansuri) canbe understood as vibrations in membranes, strings and organ pipe.

� Sound Navigation and Ranging (SONAR) and Radio Detection and Ranging(RADAR) are two techniques which have many applications. They make useof sound and electromagnetic (radio wave) waves respectively. SONAR is moreuseful than RADAR in water as electromagnetic waves lose energy fast in water.

� The inventions of microphone, speakers, telephone, satellite, computer andinternet and HAM have revolutionized communication. They all work throughthe conversion of sound wave/text into electromagnetic waves at transmissionend and reconversion to sound wave/text at the receiver’s end.

� A microphone (mic) converts sound into electrical signal, while the speakerconverts it back into sound. Mic can be of different types like condenser,piezoelectric, contact and magnetic mic.

� Sound pollution can have dangerous implications and hence care should beexercised that the level is kept low. Prolonged use of mobile phone can damageus and there is possibility of serious illness.

TERMINAL EXERCISE

1. Fill in the blanks:

(i) Sound travels at a ……………… velocity than light.

(ii) When there is lightning, we first ……………… and then hear it.

(iii) SONAR makes use of ……………… waves while RADAR makes useof ……………… waves.

(iv) Microphone converts sound into ……………… while speaker convertselectric signal into ………………

2. Multiple choice type questions

(i) Which satellite will see a wider area on Earth?

(a) A low earth orbit satellite (b) A high earth orbit satellite

(c) A medium earth orbit satellite

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(ii) India’s first self launched satellite was

(a) IRS (b) Aryabhat (c) Rohini (d) INSAT

(iii) For the same velocity, will a higher frequency of a sound wave mean

(a) Higher wavelength (b) Lower wavelength

(c) The same wavelength

(iv) Sound travels fastest in

(a) Solid (b) Liquid (c) Gas

(v) The most suitable medium for RADAR would be

(a) Gas (b) Liquid (c) Solid

3. Why can’t we hear each other on Moon?

4. Describe 2 experiments to show that sound has vibrations associated with it.

5. What is the relationship between velocity, wavelength and frequency?

6. State 3 differences between sound waves and micro waves.

7. What are the differences between longitudinal and transverse sound waves?

8. Will sound move faster in solid or air?

9. What is the basic difference between noise and music?

10. What makes your voice appear more musical when you sing in a bathroom?

11. How is active SONAR different from passive SONAR?

12. What are the relative merits of SONAR and RADAR? Why is it better to useSONAR in water?

13. How does SONAR help in estimating the distance of an object?


18.1

1. The wave with frequency 100 will have its crests farther apart as its wave lengthwill be higher. For sound waves, the velocity ‘v’ is equal to the product ofwavelength and frequency (v = n × λ or v/n = λ) and thus wavelength andfrequency are inversely proportional. So for the same velocity, the lowerfrequency wave will have a larger wave length. Therefore, for the wave with afrequency of 100 Hz, will have a higher wavelength and the crests will be fartherapart compared to the wave with frequency 500 Hz.

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2. Wavelength = 0.33 meter

3. About 20 Hz to 20KHz

18.2

1. A wave transfers energy. Even when the material is displaced, it’s temporary andit comes back to its normal position such as in case of a ripple in water.

2. Medium is essential for propagation of mechanical waves. Electromagnetic wavescan travel through vacuum as well as any medium. But they lose energy in liquidsand solids very fast. Sound waves can travel through liquids and solids with muchlower losses. The velocity of sound waves is highest in solids (a few thousandmetres per second). The velocity of electromagnetic waves in contrast isextremely high: about 3 lakh km per second.

3. In a transverse wave, the direction of propagation of the wave (the direction ofenergy-transfer) is perpendicular to the direction of oscillations whereas inlongitudinal waves, particles of the medium vibrate parallel to the direction ofwave propagation.

4. Yes. The sound waves travel in solids.

18.3

1. The unit to measure sound level is decibel. It’s one tenth of a bel. Actually thedecibel is a comparative scale. For us, the reference is fixed at the just audiblesound so we normally speak of sound level in decibels.

2. Flute is an organ pipe in which air columns vibrate. More the length of the aircolumn, more the wavelength of sound produced and hence loss the frequency.Holes are provided by the side of the flute so that by closing the holes lengthof vibrating air column may be changed.

18.4

1. Telephone, Radio and Television

2. In a condenser microphone, if the diaphragm is made very heavy, the inertia ofthe diaphragm will be higher. This means it will be difficult for the diaphragm tomove rapidly. Its movement can’t be fast enough and so it will not be possibleto reproduce very high frequencies.

18.5

1. Satellites are useful in communication, surveying, photographing of geographicalfeatures of earth and astronomy.

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2. If the satellite is stationary but earth below it keeps moving, the view will keepchanging. Thus without the satellite moving, the satellite cameras will see wholesurface of earth facing it.

3. The geostationary, polar and low satellites. The geostationary satellites are thehighest at about 36,000 km. The Polar satellites are lower than them and theLow Earth Satellites (160-1400 km) are the lowest.

4. Geostationary satellites are preferred for communication application. This isbecause from earth they appear fixed at the same place. Thus if the antennasare directed towards them once, we don’t have to worry about tracking them.

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